By Jessica DeWitt and Geoff Cunfer
This lesson introduces the concept of digitizing within ArcGIS. The digitization of historic maps makes it possible to extract features from the original, paper map and bring them into a GIS. Key concepts will be represented in bold. For this lesson we will be using a map scanned by Geoff Cunfer, Professor of History at the University of Saskatchewan, during a research trip to the archives of the Kansas State Historical Society in Topeka, Kansas. The archive holds copies of landownership maps from across the state, but does not allow researchers to handle the originals. Instead, they make them available on microfilm. Cunfer printed the image from a microfilm machine, then scanned it with a flatbed scanner. The map’s main purpose is to show who owns which parcels of land in Logan Township, Rooks County, Kansas, and it dates to 1905. “Plat maps” like this one are important artifacts of the American property rights regime and are common across North America. They contain rich information about social structure, class, land use, and many other aspects of rural life.
Download ArcLesson4.zip and save in your Documents\ArcGIS folder (make sure you unzip the folder). The final file path should be YOURUSERNAME\Documents\ArcGIS\ArcLesson4
1. The first step is to examine your image. -In an image viewer, open your image. For this lesson we are using the image Plat_LoganTwp_1905.jpg located in the Workshop 3, Scans Folder. 2. Look at the map closely using the zoom and pan tools. 3. While looking closely at the map start identifying the features that can be digitized into separate map layers. 4. What map features can you identify in this map? -Today we will focus on three map features found on this map: land parcels, buildings, and the rail line.
The land parcels are the polygons identified by the owner’s name and the parcel’s acreage. -The buildings are represented by the black dots on the map.
The rail line is represented by the dashed, dark line shown in the above image.
*Remember vector map data can be represented by three different GIS data types: points, lines, and polygons. See ArcGIS Lesson 1.
5. Now identify what type of data will best represent our three select map features.
- The land parcels will be created using a polygon layer.
- The buildings will be created using a point layer.
- The rail line will be created using a line layer.
6. Next, it is important to identify the map attributes that will be attached to each map layer. -What map attribute information can you identify in the map?
*Attribute Data consists of descriptive information that can be attached to a specific geographic location. Examples of attribute data include the medium income of a state (polygon), the speed limit of a roadway (line), or the name of a city (point). Each map includes its own specific attribute data.
Important attribute data in this map includes, but are not limited to:
- The acreage of each parcel of land.
- The name of the land parcel’s owner.
- The name of the railway line.
- The names of bordering townships.
- Can you identify any other attribute information?
After adequately examining the image, it is time to close the photo viewer and open ArcMap.
Import and Georeference the Map in ArcMap
7. Before a map can be digitized, it needs to be georeferenced.
- Copy and paste the scanned map, Plat_LoganTwp_1905.tif, into the GeoreferencedScans folder. When georeferencing images it is best to keep the original images and georeferenced images separate, creating a separate folder makes this easier.
- Open ArcMap and double-click the Add Data button .
- In the Workshop3 folder find the Digitizing1.gdb, double-click on it and select Sample_Townships_1905.
- Add the Plat_LoganTwp_1905.tif.
- Georeference the map image using the Sample_Townships_1905 layer. (Refer to ArcGIS Lesson “Arc 3” for georeferencing instructions.) When georeferencing the image make sure to use outer most border. The thin line along the border of the parcels is a road allowance.
- Find Logan Township, Rooks County by right clicking on the Sample_Townships_1905 layer and going to the attribute table.
- Find Logan County, Rooks County in the attribute table and click on it to highlight both in the attribute table and on the map.
- After georeferencing the Logan Township map, close ArcMap
Digitize Map Layers
8. Re-open ArcMap, the screen will be blank.
9. The Digitizing1.gdb geodatabase contains 3 feature classes to which we will now add map features. Using the Add Data Button add Rooks_Logan_1905-buildings, Rooks_Logan_1905_parcels, and Rooks_Logan_1905-rail.
10. The next step is to set the symbology for each layer. Under each layer click the symbol (Green Diamond in Below Example)
And Set to the Following:
Rooks_Logan_1905_buildings: A Point layer- Square 1, 8 Point, Red
Rooks_Logan_1905_parcels: Polygon Layer, No Fill, Outline Width 1.00, Red
Rooks_Logan_1905_rail: Railroad, Width 1.00, Red
When Working with Black and White Imagery, Red Features Provide Easily Discernible Contrasts
11. After you finish setting each feature class’ symbology, use the add data button to add the Plat_LoganTwp_1905.tif image from the GeoreferencedScans folder. Right click on the layer in the Table of Contents and click “Zoom to Layer.” Make sure the Editor toolbar is active, if not, go to the Customize Menu->Toolbars->Editor to activate it.
12. We will now begin digitizing. The first layer we will edit is the buildings layer. In the Table of Contents select the Rooks-Logan_1905_buildings layer. Then click on the dropdown menu on the Editor toolbar and select “Start Editing.”
You may encounter this warning:
This is not an issue that we need to worry about. Click “Continue”
The following “Create Features” Window will appear:
Make sure that the buildings layer is selected and that the “Point” is selected in the Construction Tools at the bottom.
*The Editing feature is quite useful, but can also be dangerous. Usually map features are locked, but once you choose to “start editing” any map feature can be added, changed, or deleted. Only use this tool when absolutely necessary. When finished editing make sure to choose “Save Edits” in the Editor Toolbar Dropdown Menu. If you make a mistake while editing there are two ways to reverse it. If only one mistake has been made, one can use the undo function to delete the most recent features created or edited. Another option is to choose “Stop Editing” in the Editor Toolbar Dropdown Menu and choose NOT to save the edits, which will revert the layer back to its original state or last save. Once your edits are saved there is no way to undo the changes that you made.
13. We will now digitize the buildings located in Sections 25-36 on the map. Zoom to Section 25.
*Sections are 1 mile square and comprise 640 acres. The section number appears in the center of each section. Section 25 is comprised of 2 quarter sections in the south owned by Goodman (160 acres) and Smee (160 acres), and 2 parcels in the north owned by Novotny (80 acres) and Baumgartner (240 acres).
-Now digitize the three buildings in section 25. The buildings are the small, black rectangles. Make sure buildings layer and point are selected in the Create Features window as discussed above. Click in the center of each building rectangle with your cursor. Each click creates a separate point in the class feature. The points should look like this:
*If you make a mistake you can use the black arrow tool on the Edit Toolbar to select a point. Once selected you can move the point to its proper position or get rid of it by hitting the “delete” key.
14. We will now digitize the Union Pacific railroad that crosses Section 31-36 into a line layer. Zoom in on Section 35 and 36 using the magnifying glass:
Click on the right-most-end of the railway line, then click left along the line in intervals, making sure to follow it as close as possible. When you get to the edge of the screen use the pan button to move leftward (Shortcut: if you use a mouse with a roller wheel, pushing the roller wheel will active the pan button automatically). When you click on the line tool again you will be able to continue your line without interruption. Continue following the line until you reach the left-most-end, then double-click to complete the line. Save your edits and stop editing. Your line should look like this:
*A line is constructed in GIS with a beginning point, a number of intermediate nodes (called vertices), and an end point. There is no specific number of vertices that a line should contain. There should be a balance between time effectiveness and accuracy. Vertices should only be added when the feature being traced has an identifiable point of inflection. The more vertices, the more accurate a line is, but it will take longer to create. The necessary accuracy depends on the scale in which the map will be used. Make as few vertices as necessary to create the accuracy needed for the scale of the map.
*Lines should be digitized in the direction that they flow. For the above railway line it does not matter because the railway travels both ways. However, if you are mapping a river or the migration of a group of people, the line should be created in the direction of the flow or movement.
15. We will now digitize the land ownership parcels in Sections 23, 24, 25, 26, 35, and 36. Zoom to Section 36.
In the Table of Contents select Rooks_Logan_1905_parcels then select “start editing” in the Editor Toolbar. In the Create Features Window make sure the Rooks_Logan_Parcels layer is selected and the polygon tool is selected in the construction tools.
On the Mayhew parcel, click on the bottom right corner. Then click on the top right corner, and the top left corner. On the bottom left corner, double-click to complete the polygon. Save edits and stop editing.
Go to the Editor Dropdown menu and select Snapping then Snapping Toolbar. Make sure all four symbols on the Snapping Toolbar are highlighted. The snapping tool pulls your cursor to an existing vertex or edge when you are close to it.
Now digitize the Smee parcel, using the snapping function to snap to the edge of the Mayhew parcel. Continue digitizing the parcel polygons in the specified sections (Sections 23, 24, 25, 26, 35, and 36). Note that property lines within sections are adjacent, however those in separate sections are not due to road allowances. When digitizing your snapping tool, depending on snap tolerance, might snap across road allowances. If this occurs, zoom in until the snap tolerance is surpassed. When finished save your edits and stop editing.
16. You have now digitized a selection of the point, line, and polygon features on the map. Explore what you have created by checking and unchecking features in the Table of Contents. Uncheck the Plat_LoganTwp_1905.tif layer
When you uncheck the layer you can see the feature class layers that you just created. It should look like this:
*Remember: Digitizing is labour-intensive, only digitize unique map features that will enable you to answer questions connected to your research and are not available elsewhere. For example, one would not digitize the township border represented on this map because this data is widely available elsewhere. Someone else has already done the work.