Intro to Atlas GIS 2.0
Mackenzie, Tanjuakio and Sparco
Intro to Atlas GIS 2.0
Digitizing involves using a digitizer tablet to create digital data files from paper maps. It is important to understand how to digitize, so that you can avoid having to do so whenever possible. Digitizing is time-consuming, expensive and boring, and you should generally consider it a last resort. Always try to get the data you need in digital form first (from TIGER, DLG or other sources). Or ask around: some other GIS shop may already have what you want.
If you do wind up having to digitize your own stuff, here are some general strategies:
Atlas GIS offers two alternatives for digitizing. You can add features to an existing geographic file, or create a new geographic file in which to digitize features. To create a new geographic file, use File-Geographic-New. Choose the projection which matches that of the paper map; otherwise, you will receive large errors when transforming the control points and your map will not be accurate. If the map you want to digitize off of does not specify its projection or coordinate system, you may have to pick a user-defined system. Note, however, that you won't be able to merge or overlay this file with geographic files in any different coordinate system! Alternately, you may be able to digitizing a map with an unspecified coordinate system into any standard projection (lat-long) if the map covers a small geographic area.
The File-Geographic-New pop-up menu asks you for a brief file description and the coordinate system (hitting the space bar in the coordinate system field will list all the projections available in Atlas GIS). The initial map view fields can now be entered. Atlas GIS will ask you to specify a Center Longitude, a Center Latitude, and a Diameter. These fields only provide a point of reference for the initial map view and are not important if you will be setting control points (see below). Atlas GIS will automatically reset the map when control points are entered.
It is generally preferable to digitize into an existing geographic file, provided the existing file has adequate resolution for your purposes. For example, you can import TIGER/Line files to create useful base map files in lat-long for counties or groups of counties (see Chapter 5).
It is often easier to read geographic coordinates for your control points (see below) from the existing file rather than having to determine them from the source map. AtlasGIS requires you set control points in lat-long: if your base map geographic file isn't already in lat-long, use File-Geographic-Tools-Project to transform it. You can re-project it again later.
First, tape the source map securely to the digitizing tablet. A poorly-secured map will shift slightly as you work on it, generating digitizing error. If the source map is too large for your digitizing tablet you can either shrink the map using a reducing photocopier (but be aware that many photocopiers have noticeable lens distortions!), or divide the map into sections and digitize the sections individually.
Next, set the tablet mode with the View-Tablet-Mode command. Select from "On-Template Enabled" (reserving a portion of the tablet for a command template), "On-Template Disabled" (activating the whole tablet for ditizing), or "Off" (for mouse or keyboard input only) in the pop-up menu. For digitizing in the Spatial Analysis lab, choose "On-Template Disabled."
Now mark 5 to 8 control points on the source map. It is a good idea to write in a descriptive name for each point so you can keep them straight later on. AtlasGIS will use these control points to calculate (by regression analysis) an equation by which it translates every coordinate input from the tablet (in an arbitrary X-Y coordinate system) into a real-world (lat-long) coordinate in the geographic file. USGS quad maps have geo-referenced corner points and 4 interior geo-referenced control points as well; on other maps you may choose road intersections or other landmarks which are easily geo-referenced.
The View-Tablet-Control command calls up an entry screen in which you enter and analyze these control points. For each control point marked on your source map, enter the descriptive name ("NWCorner" for example) and the point's geographic coordinates (lat-long) in the appropriate columns of the entry screen. If you are digitizing into an existing geographic file, you can determine these coordinates from the geographic file using Select or Edit-Geographic-Add utilities: move the cursor to each control point, and its coordinates are displayed on the right side of the screen. Write these down, then escape out (don't actually select anything or create any new feature). Enter the longitude and latitude coordinates in the control point spreadsheet.
Now digitize each control point: highlight the appropriate control point in the spreadsheet; then position the cursor as accurately as possible on the marked point on the paper map and simply click the left cursor button. This enters the arbitrary X-Y tablet coordinates (in inches) in the screen's X and Y Tablet columns.
Now use /Tools-Transform to compute the tablet-to-map transformation equation and display each point's mean-squared error in the Tablet Error column of the control point spreadsheet. The transformation error is the difference between the map coordinates you entered and those calculated by the estimated transformation equation. Try for errors that are all 0.01 (inches) or smaller. High errors may be caused by mixed-up control points or sloppy ditizing (both correctible) or wrong map projections. A control point with an error of 0.02 or greater should be re-digitized so that your digitized features will be as accurately geo-referenced as possible. After re-digitizing control points, do another Transform so that new error figures can be calculated.
Save the settings in a control point file (/Tools-File-Save). If your ditizing project is interrupted, you can continue it at a later date by simply replacing the map on the digitizing tablet, re-digitizing the X-Y points and re-Transforming.
Once your control points are established and the Transform utility has yielded acceptably low errors, hit the F10 key or <<Done>> to return to the main menu. The maparea will display the control points as circles with cross-hairs.
The actual digitizing is done via Edit-Geographic-Add, and involves three basic steps: first, choose the layer to add features to, or create a new layer; then enter primary and secondary names and any other attributes for the feature; then digitize the feature. (Remember: only lines can be added to line layers, regions to region layers, and points to point layers. If you try to add a line to a region layer, Atlas GIS will automatically "close" your line to make it a region or polygon.)
For each new feature you will see a pop-up screen with _ID, _LAYER, _NAME and _NAME2 fields. Atlas GIS automatically assigns a unique ID to the feature, and there is usually no reason to change this or the _LAYER field entry. It is usually easier to fill in the _NAME and _NAME2 fields now rather than go back later and try to identify each one after all the features are digitized.
Digitize each feature one at a time by moving the cursor to a starting point on it and entering its successive vertices, point by point, with the left cursor button. You will see your points appear on the screen. If you make a mistake, you can delete vertices one at a time (most recent first) with the Delete key. To continue digitizing, just follow the outline of your region or line with the mouse and continue to click button #1 to create vertices.
When you digitize the last vertex of a line or region feature, hit ENTER. A command menu will then appear at the top of the screen with the following choices: Island, Another, Done, or ESC. Choose "Island" if you want to include an exterior island with (or exclude an internal lake from) the same feature. "Another" starts another feature in the same layer. ESC discards the feature you just digitized.
While digitizing is generally done freehand, there may be situations where you want to place vertices at precise distances and/or orientations from each other. The F5 key lets you set vertices at absolute locations (lat-long); the F6 key lets you locate vertices by distance and azimuth relative to the current cursor position.
When digitizing a new feature, you can make it share vertices with existing features with the Grab and Common tools in AtlasGIS's /Tools menu. The Grab tool lets you grab nearest individual vertices of existing features, and the Common tool lets you grab entire adjacent features. Note that these tools grab vertices or common shared boundary segments from existing features in any layer in the geographic file. (You can use the "6" button on the digitizing puck to Grab an existing vertex, or the "7" button to Common an existing border.) These tools highlight common vertices of existing feature (in any layer), place the cursor at the first common vertex, and usually ask you to confirm whether you want this common border or another. (If you digitize all region features in a clockwise direction, the first choice of common border you are offered should be correct.) When you are prompted to move the cursor to the last vertex in the common border, move the cursor to the last vertex and click button #1 of the mouse. When you have finished digitizing the feature, hit ENTER.
Unfortunately, AtlasGIS's Grab and Common digitizing tools aren't as efficient as they might be. When you are digitizing adjacent region features, the Common tool doesn't determine how much of a common boundary segment you might want, and generally gives you the entire existing feature--take it or leave it. On the other hand, Grabbing each individual vertex in a common border can be very time-consuming.
When using the Grab and Common tools on common borders, generally don't want to skip over any vertices. Unfortunately, AtlasGIS's default setup doesn't display individual vertices in draft mode. You can get the individual vertices of existing features to display on the map in draft mode if you go into the "Draft Mode" section of the Configure-Display menu and specify that vertices should be visible.
Digitizing points and lines is generally pretty straightforward, but digitizing regions is not. Any cluster of region features contains interior boundary segments which are shared by two adjacent features, as well as exterior segments bounding a single feature. When digitizing shared border segments, try not to redigitize segments you have already digitized for a prior feature, so you don't create gaps or overlap slivers.
This requires some strategy. You may want to digitize the individual boundary segments of region features as separate line features. Then form the region features one at a time: first, select and copy the boundary line segments defining a region to a temporary line layer, and Edit-Geographic-Union these to a single closed poly-line feature. (If you Union boundary segments in the original line layer, you can't use them a second time for adjacent features.) Then Edit-Geographic-Move the poly-line to a target region layer. (If you don't Union the segments first, you get a collection of weird regions rather than the one region.) Repeat for each region.
This requires a lot of repetitive keystrokes, but you can write a macro to speed things up tremendously. See Part 9 for a sample macro which does this.
Here's another digitizing strategy for a slightly more complex problem. Suppose you are digitizing multiple contiguous region features within an existing boundary (region) feature. As before, you digitize each interior boundary segment as a separate line feature. The Common tool probably won't work (how would AtlasGIS know how much of the boundary you want?), but you can Grab individual vertices from the exterior boundary (the original region feature).
Another approach would be to extend segments which touch the outer boundary beyond the outer boundary. Then connect these extended segments with extra line features outside the original region. Copy segments to another line layer, union them, and copy them to a new region layer. Finally, use Operate-Split to cut the new region features by the original boundary feature (or vice versa) to obtain the regions you want; discard junk features outside the original region boundary. (Note: the Overlay layer in Operate-Split has to be regions. Edit-Geographic-Split lets you split regions by overlaying line or region features, but we have found this utility doesn't always split regions as we expected.)
Digitize the five major watershed regions of Sussex County, Delaware.
Same objective: digitize the five major watershed regions of Sussex County, Delaware. This time you can insure that these are perfectly consistent with each other and with the county boundary in the existing geographic file (from TIGER/Line data).