Constructing and Sharing Maps

---

Beginning with ArcMap

The main ArcMap window is divided into two panes:

The larger pane on the right is the map display area. You can display geographic data here by adding it with the yellow-and-black button  Add Dataor menuing File > Add Data >  Add Data…. It’s located in the Standard toolbar, which is usually docked just below the menus in the ArcMap window.

A map of the United States including Alaska and Hawaii should now appear in the map display pane. Data such as states that are displayable geographically are called layers, because they overlay each other like transparencies when you add them. Maps can display several different kinds of layers: points, polylines, polygons, images, and others. The layer states consists of polygons defining the boundaries of the fifty states. Each of these polygons is called a feature of the layer.

The layer’s name will also be listed in the pane on the left, which is called the Table of Contents. By default its name is the data file’s root name, but it can be easily changed by cllcking on it and typing.

Experiment: Change the name of the layer  states by clicking on it, pausing, and then typing something else, e.g. United States.

The Table of Contents provides three views of your data. The primary view is  List by Drawing Order, which shows just the simple names of data layers, corresponding to what is visible (or potentially visible) in the map pane. The secondary view is  List by Source, which lists the full path names of your data files. In addition to data layers, it also shows data you’ve referenced that is undisplayable, such as tables of additional data. Usually you’ll want to stay in the Drawing Order view. The third view, Selection, is a topic for another day.

Experiment: Click on the tabs Display and Source to observe how the view of your data changes.

---

ArcMap Documents

Before we proceed any further, it’s a really good idea to save your map.

ArcMap documents preserve your current arrangement for use at a later time, and are especially useful after the occasional ArcMap crash.

In addition, ArcMap is aware of the location of a saved map, establishing it as the Home folder so that related files are easy to find.

You’ll therefore want to continue to save them on a regular basis, for example every time you’re satisfied with the current view.

ArcMap documents do not actually contain the data you add to them, such as the layer  states. Instead, they contain pointers to your data, as well as information about how to display them. That’s why it’s a good practice to keep them together, unless the data is in a standard, shared location such as an archive.

Because ArcMap documents only contain links to your data, another good practice is to make those links relative to the map document. For example, the data might be referenced as being “in the same folder as me” rather than being “in the folder C:\Documents and Settings\username\Desktop\constructingmaps”. Either of these is known as a path to the data; the former is called relative while the latter is called absolute. Relative paths will facilitate moving the folder containing your map file along with all the files linked to it to another location.

Note that you can only open ArcMap documents by double-clicking on them in the Windows Explorer, or by opening them from within  ArcMap through the menu File and the menu item Open (or clicking on the button  Open). They may not be added to a map using the button  Add Data, which is reserved for the different pieces of data that make up a map.

---

The Tools Toolbar

The primary toolbar is called Tools, and it contains the buttons listed below. They provide quick ways to zoom in and out, pan across the map, restore a map to its full extent, and return to previous views of the map:

Button	Action
	Zoom in: click a point to zoom in to it by 50%, or click-and-drag a rectangular region to view.
	Zoom out: click a point to zoom out from it by 50%, or click-and-drag a rectangle to contain the current view.
	Zoom in to current center by 20%.
	Zoom out from current center by 20%.
	Pan (drag) the map.
	Full extent: zoom out to the full map view.
	Go back to previous map view — if you lose your map, this will bring it back.
	Go forward to next map view.
	Select features.
	Clear selected features.
	Identify features.
	Find features.
	Go to an (X, Y) coordinate position.
	Measure distance along a path you define with a series of clicks (double-click the last).

The toolbar is shown above docked between the two window panes, but may be located in some other position when you open ArcMap for the first time.

Experiment: Locate the toolbar and drag it into position between the two panes (unless you prefer it somewhere else).

Experiment: Try zooming in and out of the map, panning, and using the full extent button to return to the original map view. Also try using the back and forward buttons to move along the sequence of views you’ve created.

We’ll talk about the other tools later.

---

Map Scales

The degree to which one has zoomed in to or zoomed out from the map is commonly expressed by comparing a distance on the map to the same distance in the real world. So, for example, the distance from New York to Los Angeles is about 6 centimeters in the computer view above, but roughly 4,000 kilometers in real life. Since 6 cm = 0.00006 Km, we can calculate a ratio of these two numbers that doesn’t depend on units, 0.00006 Km : 4,000 Km = 1 : 70,000,000. This ratio describes what one map unit corresponds to in the real world, and is called the map scale.

ArcMap displays the current map scale in a text field / menu at the top of the screen just to the right of the button  Add Data:

Intially the scale for your map will be around 1 : 100,000,000, depending on the size of the map display pane. As you use the zoom buttons in the Tools toolbar, the scale adjusts automatically. It’s also possible to change the map scale by clicking and typing directly in the text field, or by clicking on the adjacent pop-up menu button and choosing from a list of common values.

Experiment: Observe how the map scale changes as you zoom in and out of the map. Try changing the scale by typing a number in its text box, and by choosing a value from the menu.

Note that as you decrease the second number in the map scale ratio, the scale increases. Cartographers therefore use the following terms to describe scales:

At a small scale: the ratio is small; the second number is large; the map is zoomed out; you can see a large area; you see less detail.

At a large scale: the ratio is large; the second number is small; the map is zoomed in; you can see a small area; you see greater detail.

---

Tables, Records, and Fields

When you add a map layer, you will also bring along an attribute table describing the individual features of the map. For example, every state in the map has a name, and we may want to label them, so we need to know how those names are associated with the polygons.

Shortcut: you can also open a layer’s attribute table by holding down the Ctrl key and double-clicking on the layer’s name in the Table of Contents.
An attribute table appears in its own window floating above the map, and it looks something like the following:

In this table, every feature of the layer is listed in its own row or record. Each column or field represents a different attribute of these features. So, in this case, we see each state name on a different row, along with its population in 2000 and 2005, its federal information-processing code, etc.

Every attribute table will begin with the two structural fields FID and Shape. The first is a Feature Identifier that will always be a unique number to distinguish one feature from another. The second is a summary description of the geography of this feature; hidden behind the text “Polygon” there’s a lot of information about how to draw the lines that comprise it.

All other fields are optional, but their presence is important to enable the true power of GIS. These fields are usually one of two types:

• Text, such as STATE_NAME; these field values are left-justified.
• Numeric, such as POP2000: these field values are right-justified.

Note that sometimes fields appear to be numeric but in fact are text, e.g. STATE_FIPS, because they are stored as a sequence of individual digits.

---

Locating Attribute Table Records

The features in a table are often in a random order. However, you can compare them more easily with each other by sorting them by any one of the attributes in the table.

Note that all of the data in a row stays together, even though you’ve sorted on a single column (something that doesn’t automatically happen in a program like Excel); this is a common feature of a database.

Experiment: Sort the data by different fields in the table.

---

Locating Map Features

Once you have found a feature in a table, you can locate it on the map in a few ways.

The most common is to select it, but you can also flash it and zoom to it.

---

Selecting Map Features

When you don’t have the attribute table open, and you recognize features on the map, you can also select them using the tool  Select Features.

---

Identifying Map Features

When you don’t have the attribute table open, a quick way to get information about a particular map feature is to use the tool Identify.

---

Finding Map Features

More often than not you won’t recognize a feature on the map, but you can find it without going into the attribute table using the tool Find.

---

Symbolizing Polygons Using Categories

Right-click on the map layer again and select Properties to bring up the Layer Properties screen. Now select the Symbology tab.

At present, the map is colored using a single color (symbol) for every feature (state polygon). If we simply wanted to change this color, we could click on the colored rectangle and select a different color. However, what we want to do is color the states different colors. To color the states different random colors, select Categories (which will bring up a new screen), set the Value Field to STATE_NAME--because we want to give different colors to states with different names. Then choose a Color Scheme on the right that consists of patches of distinct colors.

Click the Add All Values button and OK. Each state will now be colored by one of the colors from the scheme you chose. You can go back to the original single color for all states through the Symbology tab by selecting Features then Single Symbol then OK.

Checklist for Coloring states random colors Right-click the states layer and select Properties Click the Symbology tab Select Categories Select STATE_NAME as the Value Field Select a Color Scheme with patches of colors Click Add All Values

---

Symbolizing Polygons Using Quantitative Fields

Right-click the states layer, select Properties and then the Symbology tab. Select Quantities on the left.

Begin by selecting the quantitative variable for coloring the map as the Value in the Fields section.

Coloring a map using a quantitative variable is more complicated than coloring it using random different colors. There are three basic choices to make:

1. You need to select a color scheme from the ones provided. Generally you will select a color ramp whose values vary in a continuous way to indicate change in the quantitative variable.
2. You need to decide how many value classes you want to use. This will group the values into a discrete set of ranges.
3. Finally, you need to decide what method you will use to classify the values (choose the value ranges). ArcMap provides several ways to do this automatically, as well as the option of letting you set the break points between categories manually.

Such a map is sometimes called a heat map because of its common use to describe temperature, though usually with a blue-to-red (cold to hot) color ramp.

---

Classification Methods

ArcMap will by default use five categories and the natural breaks method for dividing the values into five categories. The natural breaks method tries to draw the lines between categories of values where they naturally break. If you want more categories, you can easily change the number. Usually between five and eight categories works reasonably well. If you use too many categories, the color values may be difficult to distinguish.

To use a different method of classifying values, click the Classify button over on the right side of the screen.

The quantile method creates categories with near-equal numbers of features in each. If we use it to create five categories of states and there are 51 states (50 states and the District of Columbia), we will get categories that each contain ten or eleven states. The equal-interval method divides the values into categories whose value ranges are the same. Choose one of these methods and click OK on this screen and then on the Layer Properties screen. The map will be colored--or symbolized according to the choices you made.

Checklist for coloring a map using a quantitative field Right-click the states layer and select Properties, then the Symbology tab Click Quantities Select the field you want to use in Field Value Select the number of Classes Choose a Color Ramp Click Classify to change the classification scheme

---

Eliminating the Size Dependence of Data Display

When looking at a map like the previous one, which displays population as a color spread out over the entire state, our brains automatically integrate the area, emphasizing larger states over smaller states, even if they have the same population. Compare, for example North Dakota and Alaska —  which one looks like it has the larger population? They actually have the same population, with only a 2% difference.

So, instead of coloring a map with a simple numeric attribute such as “total population ” — an extensive quantity — it’s usually better to divide it by the state’s area to map population density — an intensive quantity. It’s also reasonable to map the ratio of any two extensive attributes where that makes sense, e.g. “people over 80” divided by “total population”, to find the relative fraction of an attribute, which is also an intensive quantity.

In either case, this is easily accomplished in ArcGIS by specifying the field in the denominator as a normalization field.

Such a representation of data is known as a choropleth map, from the Greek for khōra (region) + plēthos (multitude).

Both the classification and normalization of your data can make major changes in the way your map looks and in the way it’s interpreted by someone who neglects to read the legend. It’s important to make these choices based on an understanding of the data, with an eye on establishing visual contrast between different data groups.

---

Creating Layer Files

Once you have colored a layer in a particular way, it’s sometimes useful to save that configuration for future use, such as in another map. A layer document stores a reference to a data set and how it’s currently symbolized. It can be added to a map just like the plain data set.

---

Basemaps

Often you’ll want to provide background for your data, such as streets or terrain, and not have to worry about constructing it yourself.

The company that makes ArcGIS, ESRI, provides a large number of basemaps through Internet servers, which you can easily add to your ArcGIS map.

---

ArcGIS Online

ArcGiS Online (www.arcgis.com) is a source of geographic data in formats that are ready-to-use with ArcGIS “Desktop”.

It has datasets provided by ESRI (in particular the same basemaps as above, and others that sometimes duplicate what’s in  G:\Maps), as well as many that are contributed by others.

---

Finding Data on the Internet

It’s always worth searching the Internet for geographic data.

Much of this data must be converted into a format that ArcGIS can properly display, and working with such data will be the topic of the next few sections.

However, many sets of data are “prepared” data, i.e. they are ready to be loaded into ArcGIS and immediately displayed.

Commonly such data would be labeled as shapefiles, though they are likely to be packaged as a .zip archive that Windows will automatically open.

Another relatively new format is the geoTIFF, a standard TIFF image that’s been enhanced with geographic information.

---

Cartography

Cartography describes the clear, elegant, and even artful design of a map.

With an appropriate choice of content, structure, symbology, and labels, a map can speak volumes while still being easy to read and understand. A map that is just thrown together may be helpful to the author, who knows what to look for, but could be confusing and quickly ignored by an intended audience.

Entire books have been written on designing good maps. In lieu of reading one of them, here are some important considerations to keep in mind when preparing your maps for sharing:

• Color: Choose an appropriate symbology for your data and background for your maps that will enhance rather than detract from the visibility of your data.
  
  There are a number of color standards that people respond to perceptually, e.g. darker colors for larger numbers, and others that they expect, e.g. green for forest areas, etc.
  
  Colorblindness also prevents some people from distinguishing certain colors. So it’s a good idea to pick your color combinations carefully. Visit http://colorbrewer2.org/ for some ideas.
  
  Also note that many journals still only publish black-and-white maps, which may require simplification.
• Contrast: More generally, the different elements of your map need to be distinguishable from each other. Pick symbols, labels, and graphic element backgrounds so that they are visible when they overlap.
• Scale: Usually you will want to use a scale such that your data or important background elements have maximal magnification and are centered. If printing on paper, use the dialog Page and Print Setup… to orient the paper as portrait or landscape to match the data.
• Projection: This is a topic that will be discussed later, but in most cases it’s important to use an appropriate projection that reduces distortion over the area of interest.
• Standard Map Elements: A number of additional elements that help explain the map, such as a legend, north arrow, and scale bar, should always be included, and these will be described next.
• Titles and Captions: The names of data layers will be visible in the legend, so make sure they are not the default abbreviated names. In most cases the map itself should also have a descriptive title.
• Graphics: Other graphic elements are often useful, such as arrows to point out particular features, and can be added using the Draw toolbar.
• Inset Map: For larger-scale maps, it can be helpful to include a small-scale map as an inset to provide a locational context within a more well-know geography, e.g. a particular region within New England. Thes
  

---

Preparing a Map for Sharing

When you publish your map to static formats such as paper, digital images, or PDF, you’ll need to define a size for the output, which you can think of simply as the paper size. Menu File > Page and Print Setup..., and proceed in the usual way to choose Printer, Paper Size, Paper Orientation, etc. In addition, you’ll probably want to check the box Use Printer Paper Settings, since that is usually a good size for sharing.

To see how your map will look on the printed page, go to the View menu and select Layout View.

This displays the map to fit inside the margins of the paper type chosen. You can change the margins by clicking on them and dragging one of the blue boxes to a new position. Note the thin gray dotted line; it shows the actual limits to printing on the paper size for the printer you’ve chosen.

The map as displayed in the margins is the same one you’ve chosen in the data view, no matter how big or small the margins are. You can use the same tools in the toolbar Tools to change the relative size and position of the map within those margins. Important: in the Layout View, you should have access to the toolbar Layout, which provides similar tools that reference the paper rather than the map — with them you can zoom into the paper without changing the size of the map relative to the paper.

Before sharing your map, it’s a good cartographic practice to add a title, a legend to describe the different layers, a north arrow to show directions, and a scale bar to show the map scale:

1. Go to the menu Insert and select the menu item Title, and a text box will appear near the top center of the screen. Enter a title and drag the box to an appropriate position.
2. Next go again to the menu Insert and this time select the menu item Legend. Take all the defaults by clicking the button Next on each of the series of screens. Drag the legend to an appropriate place and resize it if necessary.
3. Now menu Insert and select the menu item North Arrow...; choose your preferred style, click the button OK, and then move the arrow where you would like it.
4. Finally menu Insert and then select the menu item Scale Bar...; choose your preferred style, click the button OK, and then move the scale bar where you would like it. Note that by default the scale uses whatever the map units are; you can change that by double-clicking on the bar and choosing a different one.

If necessary, resize the margins around the map to fit these new additions.

ArcMap can have multiple data frames defined in its Table of Contents, but only one of them can be displayed at a time in the Data View.

However, layouts can display the maps from all frames at once, for example to show an overview map in an inset box.

---

Saving a Map as an Image or Acrobat PDF

The most basic way to share maps is to save them in one of the file formats commonly found on the Internet.

Go to the menu File and choose the menu item Export Map.... Then navigate to the folder in which you want to save the map.

If your intent is to use the map on a computer, such as in a web page or a PowerPoint presentation, where printing is a secondary consideration:

• If you have raster images in your map, select JPEG as the format; it will provide lossy but excellent compression.
• Otherwise choose PNG, which produces good compressed files from vector graphics (it also does a reasonable job on raster graphics).

Choose an image resolution of about 100 dots per inch (dpi), good for display on most computers.

If your intent is to use the map in a paper or a poster that will be printed, for example with Microsoft Word or Adobe InDesign, you will want to save it in an image format that preserves lots of detail; TIFF works well. Choose a resolution of at least 300 dpi.

If you would like the map to be a stand-alone document, then the Acrobat format PDF is a good choice. These files can be easily displayed by most people with the Adobe Reader software, and it’s readily downloaded by web browsers. PDF will preserve more of the details of the map, and so it’s a good way to distribute it if it will later be printed. A PDF map even provides some degree of interactivity, e.g. turning layers on and off. Important: make sure you click on the tab Format and then choose the checkbox Embed All Document Fonts; this will help keep the document readable by everyone.

Experiment: Try producing all three of these file formats, and compare them. In the PDF document, note the tab on the left called Layers; click on it and then click on the “eye” icon next to one of your layers, and observe what happens.

---

Saving a Layer for Google Earth

The free application Google Earth has become very popular, as it provides imagery of the Earth’s surface (taken from airplanes or satellites) in an easy-to-access format — if you have an internet connection.

It’s also a lot of fun to use, as you can zoom around the Earth and view it from different perspectives, including terrain and buildings in 3D!

ArcGIS is a useful program to create additional layers that can be viewed in Google Earth.

Exporting to Google Earth makes use of a special geoprocessing tool, an extra program that transforms geographic data into new formats from which additional information can be extracted.

Geoprocessing tools are stored in a collection called ArcToolbox.

The next two procedures are preliminary steps useful for most geoprocessing activities.

---

Interactive Maps

It is also possible to create an interactive map that allows the viewer of the map to explore it by zooming and viewing attributes, very similar to what you have been doing already with ArcMap.

Such interactive maps use the free program ArcReader, though it’s only available for Windows.

Creating an ArcReader map involves using the extension ArcPublisher.