Geographic
data is commonly described by pairs of X-Y
coordinates on the surface of the Earth, such as longitude
and latitude. This is the type of spatial
information that underlies the shapefiles
used in the previous two sections, but it
may also show up in simple tables. GPS receivers
also describe
your location using X-Y coordinates.
Since this tutorial will be using specific maps and data, the first step is to make your own copy of the tutorial data.
Set Up: Initializing ArcMap and Adding Data
- Start up the
 ArcMap software (see Constructing and Sharing Maps for details).
- Click on the button
 Add Data.
- In the dialog Add Data, navigate into the folder
 MappingCoordinates; if necessary, make a new connection to it (see Constructing and Sharing Maps for details). - In the folder MappingCoordinates, click on the file
 countries.shp.
- Click on the button Add.
ArcMap will now display a map of the countries of the world:
Move the cursor across the map, and notice the two changing numbers
in the lower right corner of the window (here
95°20'"W 57°49'52.523"S). This pair of X-Y coordinates
are the longitude and latitude of the tip of the
cursor, to be described next.
Geographic coordinate systems, describing positions on the surface of the Earth in latitude and longitude, are the most common representation of spatial data.
Since the time of the Ancient Greeks it has been known that the Earth was a spherical object rather than a flat surface.
Though it was suggested millennia ago that the Earth rotates once a day, this fact was not widely accepted until the 17th century, and was not firmly established until the 19th century.
The Earth’s rotation defines certain reference points and circles that we can use to determine our position on its surface.
The Earth’s rotation axis is a line that passes through the the North Pole, the South Pole, and the center of the Earth.
The Equator is a circle on the Earth’s surface that’s perpendicular to its axis and equidistant from its poles:
The reference points described above establish the four Cardinal Directions.
The direction toward the North Pole is North, and South is in the opposite direction, toward the South Pole.
The direction parallel to the Equator and toward the Earth’s rotation is East, while the direction opposite to the Earth’s rotation is West.
By definition, North and South will always be at right angles to East and West, at any point on the surface of the Earth.
In addition, the direction toward or away from the Earth’s Center are, of course, down and up, respectively.
It is useful and important to be able to precisely specify positions on the Earth’s surface: to compare positions, calculate distances, and in general navigate from one point to another.
So, a pair of numbers or geographic coordinates are used that are similar to the x and y Cartesian coordinates in a plane, but designed for a sphere.
These two numbers, latitude and longitude, are angles measuring south-to-north and west-to-east, respectively.
Any circle parallel to the Equator is called a parallel of latitude.
The angle (with vertex at the center of the Earth) between a given parallel of latitude and the Equator describes that parallel and any point on it, and is called the latitude.
So, the North Pole is at 90° north latitude, the Equator itself is 0° latitude, and the South Pole is 90° south latitude.
Amherst is located at 42.37° north latitude.
Southern latitudes are often expressed as negative values, particularly in computer applications such as GIS.
One degree of latitude corresponds to a distance of 111 Km (69 miles) across the Earth’s surface.
Any semicircle passing through the poles is called a meridian of longitude.
One of these is designated as the Prime Meridian, usually
the one passing through the Royal Observatory
in Greenwich, England (just outside London).
The angle (with vertex at the center of the Earth) along the Equator between a given meridian and the Prime Meridian describes that meridian and any point on it.
So Amherst is located at 72.52° west longitude.
Western longitudes are often expressed as negative values, particularly in computer applications such as GIS.
Note that the antimeridian can be described by either 180° west
longitude or 180° east
longitude.
One degree of longitude at the Equator also corresponds to 111 Km; but this gets progressively smaller as one moves towards the poles, eventually shrinking to zero (varying as the cosine of latitude).
Because a degree of latitude or longitude is relatively large, a common practice is to break them down into smaller units.
A minute of arc is defined to be 1/60 of a degree, often abbreviated as a single prime (').
A minute of arc corresponds to 1.86 Km = 1.15 miles (called a "nautical mile").
A second of arc is defined to be 1/60 of a minute of arc, often abbreviated as a double prime ('').
A second of arc corresponds to 31.0 m = 101 feet.
Experiment: Moving your cursor across
the map of the world’s countries, determine the
approximate location of Amherst, Massachusetts, USA.
The location
of Amherst Center is, in fact, very
accurately known.... 42° 22' 31'' N. Latitude 72° 31' 11'' W. Longitude.
- In ArcMap,
in the main toolbar Tools,
click on the button
 Go to XY (it’s
also in the menu Edit).
- The dialog Go
TO XY (Degrees Minutes Seconds) will
open; move it to a convenient location
that doesn’t obscure the map.
 In the fields Long: and
Lat:, type
in the coordinates of Amherst in Degrees-Minutes-Seconds
(DMS) format, but without punctuation:
- 72
31 11 W or -72
31 11
- 42
22 31 N or 42
22 31
- Click on one of the available tools:
 Pan to Center
the position but don’t change
the zoom level. Zoom to Center
the position and zoom in
somewhat. Flash Indicate
the position but don’t change
the map center or the zoom
level. Add Point
Add a point marker at that
location (note: this
is only a graphic, not
actual point data). Add Callout Add
a "balloon" marker displaying
the coordinates.
- Other coordinate formats such
as Decimal Degrees (e.g. -72.5197°, 42.3753°
for Amherst) are available by clicking
on the menu
 Units.
 In the seventeenth century, Isaac Newton suggested that, because the Earth is rotating and not perfectly rigid, it will bulge slightly at its equator.
So, the Earth is not precisely spherical, but instead is an oblate ellipsoid, like a squashed beach ball.
Precise measurements put the equatorial and polar diameters of the Earth at 12,756 Km and 12,713 Km, respectively, a difference of only 43 Km (0.34%).
This small oblateness can still effect the positioning of maps, so it must be taken into account.
In addition, the Earth has substantial variations in the elevation of its surface from point to point:
- The peak of Mt. Everest is 9 Km above sea level.
- The deepest point of the Marianna Trench is 11 Km below sea level.
 (4)
Because gravity depends on the mass of the Earth, there are small
variations in gravitational force across its
surface, which are reflected by local sea level
(because fluids will move in response). The geoid is
an equal-gravity surface that includes local
sea level but also continues into continental
areas, as shown in the image above. The GOCE
satellite has provided detailed measurements
of the geoid, whose variations are displayed
in exaggerated form in this mp4 movie:
 Because the Earth’s surface is so rough, fitting it in the best way with an ellipsoid depends on where you want to map it!
A datum is a choice of ellipsoid to model the Earth’s surface, viz. the location of its center, its size, and its orientation.
Many datums have been defined; U.S. maps commonly use the North American Datum of 1927 (NAD27), and more recently, NAD83.
With the expansion of international travel and commerce, worldwide
standards have been adopted, such as the World Geodetic System
of 1984 (WGS84), which is based on the geoid.
Note that this means that a measurement of latitude and longitude will depend on which datum you use!
The map at the right comparing NAD27 and NAD83 demonstrates how much measured positions can shift when switching between datums.
You should therefore always ascertain the datum when you’ve been given geographic data (e.g. NAD83 for Amherst, above).
The datum is the foundation of a geographic data set’s spatial reference; let’s look at an example:
- In ArcMap, in the Table of Contents, double-click on the layer of interest, e.g. countries.
- In the dialog Layer Properties, click on the tab Source.
- Read the text field Data Source. You should see both the datum listed as well as the geographic coordinate system.
To view the Earth on a flat piece of paper or a computer screen, its curved surface must be projected.
Once a datum has been chosen as a model of the Earth, it is straightforward to reproduce its features on a globe.
For many purposes it’s much more useful to represent the Earth on a flat surface, such as paper or a computer screen.
Such a flattened representation of the Earth is called a map.
The flattening process is known as a projection.
Map projections are similar to other projections you may be familiar with, such as projecting a slide or transparency onto a screen.
There are three common, general ways to "flatten" the Earth: Planar, Conic, and Cylindrical:
- Planar Projection: hold a plane surface up to the Earth,
and project outward in some way, as shown
below left.
  This type of projection is commonly used to represent the Earth as a globe.
This includes one common variant, the Vertical
Perspective Projection shown at the right, which is used by Google
Earth to provide a "birds-eye view" of the
Earth.
Question: What’s missing in
the vertical perspective, in comparison to
the orthographic projection
at left? This could also be described as
"something" that’s present but limiting the
view.
- Conic Projection: wrap a cone around the Earth:
 (1)
then cut it along one side and flatten it:
 (1)
- Cylindrical Projection: wrap a cylinder around the Earth:
 (1)
and again cut it along one side and flatten it:
 (1)
There are also many other, more complicated, projections that are used
for certain purposes.
Projection surfaces can be tangent to the Earth’s surface
(touching it along one standard point or standard
curve),
as in all of the images above, or secant to
it (intersecting at one or two standard curves),
as in the following images:
A large number of different projections are described in the University of Colorado’s Map Projection Overview and displayed with Penn State’s Interactive
Album of Map Projections.
For each of these surfaces, there are a number of different ways to project the Earth’s features onto them.
Any projection will necessarily distort some aspect of geography:
- distance: all projections distort distance in some way.
- shape: some projections can preserve angles and therefore small shapes, and are said to be conformal.
- area: some projections can preserve relative area, and are called equal-area.
Warning: No projection can be both conformal and equal-area.
The Mercator projection (right) is a famous example of a conformal map, in this case a coaxial cylindrical projection that
makes navigation easier by preserving directions,
but severely distorts area near the poles. Because
it also maintains shapes over
small regions, it is used by Google
Maps.
The Gall-Peters projection (below left) is an example of an equal-area map, also coaxial cylindrical, sometimes used to avoid the exaggerated area of the global north seen in the Mercator projection, but at the expense of shape accuracy away from 45° latitude.
The Plate Carrée projection (below right) is a specific case of the equirectangular projection, again a coaxial cylindrical projection that preserves (longitude, latitude) by simply mapping it to (x, y); however, it is neither conformal nor equal-area (though it is equidistant north-south and approximately along the equator).
 
Question: When bringing in data defined only in terms of geographic coordinates, ArcMap uses a default projection. Can you tell what it is?
Any distortion introduced by a projection will be smallest near the standard points
or curves where the projection surface touches
the Earth’s surface.
Non-global maps will therefore generally use a projection that minimizes
distortion in the region of interest.
Regions that are elongated east-west are commonly represented by coaxial conic projections (touching along parallels).
Regions
that are elongated north-south are commonly represented
by transverse cylindrical
projections (touching along meridians).
Regions that are not elongated one way or the other may be represented
by concentric planar projections.
 (4)
 If the map will cover a relatively wide area, secant
projections are generally used, as in the image above, since
they even out the distortion around the multiple
standard curves.
Once the map orientation is determined,
one must choose between other characteristics
such as whether it should be conformal or equal
area.
For example, here are two different coaxial
conic projections:
- Lambert Conformal Conic (right (1))
- Albers Equal-Area Conic (below, standard parallel = 45°)
The Transverse Mercator projection
(below) is a common example of a conformal transverse cylindrical
projection, in this case designed for use around
the prime meridian or the antimeridian.
(What’s that on the left side?)
Question: In this gallery
of projections centered on Latin America, which do you think
is the best choice?
It can be very important to make a good choice of projection; consider
this discussion
in a fictional White House.
GIS makes it easy to display your data with
different projections.
 The
spatial reference of the map displayed by
ArcMap is determined by the data frame, which is
indicated by the stack-of-layers icon and
the default name Layers(click-pause-click
on the name to change it).
All layers in a data frame will be projected in the same way;
essentially, it’s "the map".
A data frame’s spatial reference is initially determined by
the first layer added to it; in the Setup this was the layer countries.shp.
 In ArcMap,
in the Table of Contents,
double-click on the data frame of interest,
e.g. Layers.
- In the dialog Data Frame Properties,
click on the tab Coordinate
System.
- In the upper pane you’ll find a large collection of possible
datums and projections. Navigate
through the various possibilities
by clicking on the + and – before
their names to open and close them:
- Geographic
Coordinate Systems lists
various datums, all of which are
displayed with the default
projection.
- Projected
Coordinate Systems provides
a large collection of projections
based on different datums.
- Choose one, for example Projected
Coordinate Systems > World >
 Robinson (world).
Note that the selected spatial reference
is shown
in detail in the section Current
Coordinate System:.
- Click the button Apply to
see the effect on the map.
- When you are finished, click the button OK.
Experiment: A number of additional projections are available to display
the entire world in what are sometimes more usable formats, e.g. including Mollweide, Eckert IV, and Polar. Try them!
Extra Absurdum: What does your favorite map projection say about you?
Once the Earth is flattened, its often easiest to use planar coordinates to describe it.
 A map projection, being flat, will often be given its own set of Cartesian map coordinates.
The origin is generally chosen to be far west and south of the region of interest.
Both coordinates (x, y) then increase towards the east and north, and are therefore always positive numbers.
(x, y) are known as the easting and northing, respectively.
The origin is typically defined by the false easting and false northing, which are the map coordinates of the standard points or curves that define the projection.
Map coordinates are generally measured in linear units such as feet or meters.
State Plane Coordinates are defined by each individual state to provide a highly accurate (< 0.01%) system of mapping for surveying, etc.
Most data coming from government institutions at the scale of a state or less will be in state plane coordinates.
Current State Plane Coordinates are based on the NAD83 datum and two conformal secant projections, Lambert Conic or Transverse Mercator, and use units of meters.
 (4)
The low distortion requires state plane maps to be no more than 158 mi across, so most states use more than one projection to cover their area, breaking at county boundaries.
Massachusetts State Plane Coordinates are based on two Lambert Conic projections, one for the Mainland Zone (most of the state) and the other for the Island Zone (Dukes and Nantucket Counties — the Elizabeth Islands and Martha’s Vineyard, and Nantucket Island):
 (2)
The Universal Transverse Mercator system provides a uniform way to describe any non-polar location on the Earth with good accuracy (< 0.08%).
Most maps coming from the US Geological Survey will be in UTM.
The Earth is divided into sixty narrow north-south strips, each six degrees of longitude wide and extending from 80° S. Latitude to 84° N. Latitude:
 The zones are numbered from west to east, starting with 1 from 180° W. Longitude, and are individually mapped with a transverse mercator projection centered on the zone.
The central meridian of each zone is assigned a false easting of 500,000 meters, and the Equator is assigned a false northing of zero meters in the northern hemisphere, and 10,000,000 meters in the southern hemisphere.
Massachusetts is covered by Zones 18N and 19N (3).
As a world-oriented coordinate system, UTM is usually used with the
WGS84 datum (though not always).
UTM is also the basis of the new U.S.
National Grid system being used by
the Department of Homeland Security.
Let’s change the world map’s spatial reference
to UTM 14N/WGS 84:
- Open the data frame properties to
the tab Coordinate
System as described
in Procedure
3.
- Navigate to the
coordinate system WGS
1984 UTM Zone14N; make sure you
select the version whose linear unit
is meters.
- Click the button OK.
GIS lets you combine maps with different datums/projections/coordinate
systems, and display them with any other one
you prefer.
To accurately represent mapped data on a computer screen, and to ensure that it can successfully be used with other data, it must have a spatial reference defined for it, which includes a datum, possibly a projection, and a coordinate system.
The spatial reference determines how the map’s positions should be interpreted for display on the screen.
The spatial reference is described in a standard format that is provided
with the data in a file with the extension .prj,
and is said to be a part of its metadata (data
about data).
Sometimes the .prj file will be missing, and the spatial
reference must be manually assigned.
In order to simultaneously display two or more sets of GIS data with different spatial references, some of them must be recast to a common spatial reference.
Because each spatial reference is based on a particular datum and possibly also a projection, switching spatial references can involve a complicated mathematical process:
Switching datums is generally more complicated than
simply unprojecting and reprojecting, so approximations
are usually made that can introduce small errors.
ArcGIS has full support for multiple spatial references, and will automatically reproject data sets so that they are all displayed with the same reference.
However, because of the complexity of datum transformations, ArcGIS
(usually) will not automatically transform one datum
to another.
Instead, when data is added to a map that has a different datum, ArcGIS
puts up a dialog warning of potential issues
and giving you the option to pick a transformation.
The one exception is NAD 1927 to NAD 1983, for which there is an accepted
standard.
If a layer’s coordinate system isn’t self-described, ArcMap
assumes it is the same as that of the data
frame.
Quite often a layer will lack a .prj file, and you’ll
need to manually assign it a coordinate system.
Ideally the source will provide this information
in another format (typically just a text description).
To assign or alter the coordinate system of a layer, you must use the
 Catalog ,
which is designed for the management of individual
layers, in particular their metadata.
- To open the Catalog:
 In ArcMap,
look for the vertical tab Catalog along the right edge of the ArcMap window and point at it, whence it should automatically pop out;- If the vertical tab isn’t present, look in the toolbar Standard
and click on the button Catalog.
The Catalog will appear in its own window but can be “pinned” to the right edge of the ArcMap window, which will keep it out of the way until you need it:
- Begin to drag the window and a set of “pinner” buttons will appear; move the cursor on top of the
 right-edge pinner and release.
- Click the button
 Auto Hide at the top of the window, so that it will go away automatically when you aren’t pointing at it.
- The Catalog displays a hierarchy of data sources, starting with your home folder, viz. the folder where the current map document is saved.
Below the home folder is a list of your connected folders, followed by other resources such as database and GIS servers.
Navigate to the layer of interest,
e.g. masscounties.shp.
If necessary, make a new connection first by
going to the toolbar Standard and
clicking on the button  Connect
to Folder.
- In the left pane, click
on the file of interest, and the right pane
will display information about the
layer.
- Note the tabs above the right pane; click
on the tab Preview, and a map of the layer
will appear.
- In
the left pane, double-click on the layer
to review its properties.
 In the dialog Shapefile Properties,
click on the tab XY Coordinate System,
and note that the Current coordinate system is <Unknown>.- To set the layer’s spatial reference, you
can select one from a number of different sources:
- Another layer in the map: in the upper pane of the dialog, open the folder Layers and then click on the desired spatial
reference;
- Your set of favorite coordinate systems: in the upper pane of the dialog, open the folder
 Favorites and then click on the desired spatial
reference;
- Arc’s predefined collection of coordinate systems:
- In the upper pane of the dialog, open either Geographic Coordinate Systems or Projected Coordinate Systems, as appropriate.
- As
with data frames,
navigate through the collection
of geographic and projected
coordinate systems, and click
on the correct one, e.g. NAD 1983 StatePlane Massachusetts Mainland FIPS 2001 (meters).prj.
- Any of the above with a search term, e.g. Massachusetts, by clicking and typing in the field Type here to search, and then clicking on the button
 Search;
- Another layer not in the map:
- Click on the menu
 Add Coordinate System and select the menu item Import….
- Navigate through the file
hierarchy to find a layer
with the desired spatial
reference (often a neighbor),
and click on it.
- Click on the button Add to return to the dialog Shapefile
Properties.
The selected coordinate system will then appear as the Current coordinate system.
- If you would like to save this coordinate system as a favorite, click on the button
 Add to Favorites.
- Click on the button OK.
Feature: You can add any layer in the Catalog to the map by dragging it into the map pane; if you apply the above procedure to masscounties.shp and then drag it into the map, the Massachusetts counties should
be in their correct location.
To locate features and make measurements on a map,
you can display both the geographic and
projected coordinates.
As noted above, the location of the cursor
on the map in the current map coordinates
is displayed in the lower right corner
of the map window,
and they will change as you
move the cursor over the map.
Questions: In what
units are the current coordinates?
Where on the map are they near zero?
It’s sometimes useful to change the displayed units; as with the
map itself, this is controlled by the data
frame that holds
your layers.
- In ArcMap , double-click on Layers .
- The dialog Data Frame
Properties will now appear. Click on the tab General.
- In the area Units , in the menu Display ,
choose a unit, e.g. Kilometers.
- Click on the button OK .
Note that the displayed units will always be referenced to the origin
of the coordinate system.
ArcGIS also lets you measure the distances between locations
and the areas of regions.
 In ArcMap ,
in the toolbar Tools,
click on tool  Measure,
and the
dialog Measure will
appear. Move it around so you
can see the map pane.
- To measure
a distance:
- Click
on the menu Choose Units,
then point at the menu item Distance ,
and finally choose a unit, e.g. Kilometers.
- Click
on the the tool
 Measure Line.
- In the map pane, click
at the starting point
of your measurement,
then click on any intervening
points, and finally double-click
on the end point.
The length
of each segment will
be displayed as you go,
and the total length
will also be displayed.
- To measure an area:
- Click on the menu Choose Units,
then point at the menu
item Distance ,
and finally choose a
unit, e.g. Hectares.
- Click
on the the tool
 Measure an Area.
- In the map pane, click
at the starting point
of your measurement,
then click on any intervening
points, and finally double-click
on the end point (this
doesn’t have to be the
starting point).
The length
of each segment will
be displayed as you go,
and the total perimeter
will also be displayed,
along, of course, with
the area.
Warning: Remember that distances and
areas are usually distorted by map projections,
and can therefore have different values
in different projections (often by huge
amounts)!
Map distortion is also important when
displaying scale
bars on a layout; they
will usually only be perfectly accurate
along standard parallels and meridians,
and are best avoided if the map covers
a much larger area.
Coordinate grids are common features on maps, helping to describe the
locations of their features.
ArcMap can also superimpose
a grid on the layout view of a map, corresponding to:
- geographic coordinates, where the grid is known as a graticule,
as in the picture below;
- the map coordinates used by the
current data frame; or,
- an arbitrary set of coordinates, e.g. "A3", with which you could
create an index.
 In ArcMap,
in the menu View, select the
item Layout View.- In the Table of Contents,
double-click on the frame of interest, e.g. Layers.
- In the dialog Data Frame Properties:
- Click on the tab Grids.
- Click on the button New Grid….
- In the dialog page Grids and Graticules Wizard:
- In the area Which do you want to create?,
click on the button Graticule: divides map by meridians and parallels (the
other options are as described above).
-
In the field Grid name, you can
choose a name for the grid, to distinguish it if you create more
than one.
- Click on the button Next >.
- In the dialog page Create a graticule:
- In the area Appearance, click
on the button Graticule and labels.
The
other options don’t create a full grid but let you have,
along the edges, Tick
marks and labels or Labels only.
- In the area Intervals, type in
the spacing of the graticule lines for both parallels of latitude
and meridians of longitude (these are initially set to a suggested
value based on the map scale).
- Click on the button Next >.
- In the dialog page Axes and labels:
- In the area Axes, you can choose
whether to have both major and minor tick marks, and the
style of lines.
- In the area Labeling, you can
select the style of the text along the edges.
- Click on the button Next >.
- In the dialog page Create a graticule:
- in
the area Graticule Properties, usually
you will want to click on the button Store as a fixed grid that updates with changes to the data frame,
at least until you’re certain about the final view of the map.
The
other option, Store as a static graphic that can be edited,
won’t update automatically, but you can edit the grid
with the graphics tools, e.g. to remove specific
grid lines.
- In the area Neatline, you
can choose to have an additional border outside of the
labels by clicking on the button Place
a border outside the grid.
- Click on the button Finish.
- Back in
the dialog Data Frame Properties,
click on the button OK.
- If you later want to change some of these properties, repeat
steps (1), (2), and (3a), then click on the button Properties….
Many geographic features are described by data structures that include either geographic or projected coordinates.
Geographic
data is commonly in the form of simple
text tables describing points on
the surface of the Earth. The tables
consist of a pair of spatial coordinates
(e.g. latitude and longitude) in
each row, and possibly a feature
label and other data. Such data is
common in books and journals in all
areas of research, whether archaeology
or biology. This is also the simplest
format of data downloaded from
GPS receivers.
Tables can be in a number of different file formats but all sharing
the same simple layout, as described
in the document Mapping Place
Name Data in the section
Place
Name Data and also in the section Formatting
Tables for Joins.
For example, a collection of Massachusetts
lighthouses downloaded from the US Geological Survey can be expressed
as comma-separated values (CSV):
FeatureName,Class,County,State,Latitude,Longitude,Elev_ft
Annisquam Harbor Light,Locale,Essex,MA,42.6617614,-70.6817122,0
Annisquam Lighthouse,Locale,Essex,MA,42.6620391,-70.6825456,0
Bakers Island Light,Locale,Essex,MA,42.5364846,-70.7858796,59
Boston Lighthouse,Locale,Suffolk,MA,42.3275989,-70.8897686,0
Brant Point Light,Locale,Nantucket,MA,41.2898449,-70.0902937,0
Butler Flats Lighthouse,Locale,Bristol,MA,41.6034382,-70.8944813,0
Cape Ann Light,Locale,Essex,MA,42.6367623,-70.5750424,39
Cape Ann Lighthouse,Locale,Essex,MA,42.6373178,-70.5753201,26
Cape Poge Lighthouse,Building,Dukes,MA,41.420949,-70.4508587,23
...
Like place-name data, however, if tabular data needs to be cleaned up or processed in some other way, it is often easiest to bring it into Excel to work on it.
Important: When you compile tabular
coordinate data, make certain you note
its spatial reference! After some searching, the source for
the data above says “All coordinates in the database are in NAD 83. They were converted from NAD 27 in September 2005.”
For the following procedures, it will be useful to start with a new map.
- Review the table to make sure it meets the requirement described
in the document Mapping Place Name Data in its sections
Place Name Data and Formatting
Tables for Joins.
- In ArcMap,
in the toolbar Standard, click on
the button Add
Data.
- In the dialog Add Data,
navigate into the folder with
the table to be added, e.g. mappingcoordinates.
- Double-click on the table to be added, e.g the text file
 MassachusettsLighthouses.csv.
If ArcMap notices formatting errors
in the document, it will at this
point complain about them (hopefully
in an informative way) and refuse
to add the document.
 In the menu Tools , select the item Add XY Data….
- In the dialog Add XY Data,
in the menu Choose a table from the map or browse for another table:,
select the added file.
Note the
adjacent button  Browse,
which is an alternative to adding
the data as in Steps (2) - (4);
however, it won’t do any error
checking.
- In the area Specify the fields for the X, Y and Z coordinates: ,
in the menus X Field: and Y Field:, choose
the correct columns, e.g. Longitude and Latitude.
Optionally, you can specify the Z Field:, e.g. Elev_ft.
- In the area Coordinate System of Input Coordinates ,
click on the button Edit….
- In the dialog Spatial Reference Properties,
choose the correct spatial reference
for the data as described in Procedure
5: Steps (7) and (8) (e.g. NAD 1983).
- Back in the dialog Add XY Data,
click on the button OK .
- ArcMap may complain that The
Table Does Not Have Object-ID
Field, but you can dismiss this
complaint by clicking on the
button OK.
 In Step (4), the Table of Contents switched
its view to Source because
the table was initially not mappable.
Click
on the bottom tab Display to
see the mappable items,
and you’ll see that a new layer
was added here with the suffix "Events".
- As with geocoded
data, an events layer is constructed
on the fly, with its details
stored in the map document. It is therefore
somewhat limited in its abilities,
e.g. items are not selectable.
To
save the events layer as an independent
shapefile, right-click on the
layer to open its contextual
menu, point at the menu item Data,
then in the submenu that appears
click on the menu item Export Data….
- In the dialog Export Data,
in the menu Export, choose All features .
- In the button set Use the same coordinate system as:,
choose between retaining this layer’s source data (the
default) or using the spatial reference of the
data frame.
- Near the text field Output shapefile or feature class:
- Click on the button Browse;
- In the dialog Saving Data,
navigate to an appropriate location for the new data set,
e.g. the folder mappingcoordinates;
- Give the new layer a descriptive name, e.g. Massachusetts
Lighthouses.shp;
- Click on the button Save.
- Back in the dialog Export Data,
click
on the button OK.
- The dialog ArcMap will
now appear, asking if you want to add the exported layer to
the map; click your preference Yes or No.
The United States Geological
Survey’s Geographic
Name Information Service is a useful source of coordinate-based
data, providing an extensive list of domestic
features and their coordinates, including
many historic sites that no longer
exist. Another is the National Geospatial-Intelligence
Agency’s Geonet
Names Server, which provides
a similar service for foreign names.
These sites don’t provide their
data in an easy-to-use,
downloadable format, however,
which is also true of many other
web sites.
You will sometimes find other data formats on the Internet,
such as Keyhole Markup Language (KML) files and GPX files, which are also often produced by iPhone and Android apps.
You might also come across KMZ files, which are KML files compressed in the ZIP format — change their file extension from .kmz to .zip and you can open them and see their contents.
KML and GPX are both text
formats, but rather than being tables they are structured in a hierarchical format called eXtensible Markup Language (XML).
For example, the CSV formatted file above, with NAD83 coordinates:
FeatureName,Class,County,State,Latitude,Longitude,Elev_ft
Annisquam Harbor Light,Locale,Essex,MA,42.6617614,-70.6817122,0
...
would be expressed in KML as:
<?xml version="1.0" encoding="UTF-8"?>
<kml xmlns="http://www.opengis.net/kml/2.2" xmlns:gx="http://www.google.com/kml/ext/2.2">
<Document id="MassachusettsLighthouses">
<name>MassachusettsLighthouses</name>
<Folder id="FeatureLayer0">
<name>MassachusettsLighthouses</name>
<Placemark id="ID_00000">
<name>Annisquam Harbor Light</name>
<description><![CDATA[...Locale...Essex...MA...]]></description>
<styleUrl>#IconStyle00</styleUrl>
<Point>
<altitudeMode>absolute</altitudeMode>
<coordinates>-70.68171268410313,42.66177014248084,0</coordinates>
</Point>
</Placemark>
....
</Folder>
</Document>
</kml>
KML expects coordinates to be WGS84, hence there are slight differences from the values in the CSV data.
Recent versions of ArcGIS can add KML/KMZ files directly to your map the same as SHP files. To export, you can use the toolbox procedure described in Constructing and Sharing Maps.
Another relatively new format is Geographic JavaScript Object Notation, or GeoJSON, which is also hierarchical but (slightly) less wordy:
{
"name":"MassachusettsLighthouses",
"type":"FeatureCollection",
"crs":{
"type":"name",
"properties":{"name":"urn:ogc:def:crs:OGC:1.3:CRS83"}
},
"features":[
{
"type":"Feature",
"geometry": {
"type":"Point",
"coordinates":[-70.6817122,42.6617614,0]
},
"properties": {
"FeatureName":"Annisquam Harbor Light",
"Class":"Locale",
"County":"Essex",
"State":"MA",
...
}
},
...
]
}
Here the coordinate reference system used must be expressed in the crs property.
KML, GeoJSON, and many other formats can be read and written with the Data Interoperability Extension.
- Map Projection, Eric Weisstein’s World of Mathematics, Weisstein, Eric W., http://mathworld.wolfram.com/MapProjection.html.
- MassGIS, Bureau of Geographic Information, Commonwealth of Massachusetts, https://www.mass.gov/orgs/massgis-bureau-of-geographic-information.
- The Universal Transverse Mercator (UTM) Grid, United States Geological Survey, http://erg.usgs.gov/isb/pubs/factsheets/fs07701.html.
- The State Plane Coordinate System of 1983, James E. Stern, National Oceanic and Atmospheric Administration, http://www.ngs.noaa.gov/PUBS_LIB/ManualNOSNGS5.pdf.
- Images: Visualizing Data, National Geophysical Data Center, http://www.ngdc.noaa.gov/mgg/image/images.html#relief.
- Map Projections, U.S. Geological Survey, http://erg.usgs.gov/isb/pubs/MapProjections/projections.html.
|
K:
C:
MappingCoordinates
countries.shp
MassachusettsLighthouses.csv
Procedure
1: Locating an X-Y Coordinate Position
In the dialog 
Let’s
see what happens when you add a layer that
doesn’t have a spatial reference defined
for it to the current map:
Remove
New Map Document
