Digitizing Polygons

In this exercise, you’ll learn about creating new data that you’ll use in your next project.  We’re going to digitize the relevant parts of the BV and Glasgow Geologic maps (by Dr. Edgar Spencer, Geology, Emeritus). To do that you have to learn to “georeference” scans of maps and use the “autocomplete polygon” tool to construct the geologic polygon map.

The steps are

  1. georeference or warp ascanned image of the geologic map to the same projection as the topographic map data (using the georeferencing tools in ArcGIS).
  2. digitize the data from your tile of the map
  3. merge the tiles (after each tile is assembled)
  4. dissolve the geology units across the separate tiles
  5. Assess the precision and accuracy of this work.

To start: Copy the folder R:\courses\GEOL260\sharedwork\exercises\Exercises5\digitize to your P drive, then start a new ArcGIS project.

Warning: The map frame, data layer from which you are digitizing (topo map), and any new data layers ALL must be in the same projection.

The Geology Map

Below is what you’ll input into the group project (all are depending on you, you’ll have to pull your weight on this). The map in the background is the seamed together topographic quadrangles for this exercise (“bv&glasgow24kdrgs.jpg”) that will serve as the base for your new data.

The scans for each numbered section are in the folder R:\courses\GEOL260\sharedwork\exercises\Exercises5\geo_maps. Copy just your assigned map to your digitize folder. The pdf of the original geology maps is in the folder to use. You can see anything that doesn’t show up well in the scan (like the unit that goes with the color you’re digitizing).

Student & Map Tile number

Georeferencing the jpg image (aligning the scan of the geology map to a projected or “geolocated” drg (“digital raster graphic” or “image”) of the BV and Glasgow topo maps

  1. The georeferencing process in Pro is described here, and more generally here, but no tutorial exists, so we’ll work on that together. Lots of links to follow in the help menu (at left in the above pages) for additional help!
  2. Examine the projection and cell size information for the DRG called “bv&glasgow24kdrgs.jpg” in the digitize folder
    — Note that the digital file is not the projection of the paper maps, which are polyconic, NAD1927 datum, but how the digital files are projected in digital form.
    Note: This is two USGS 7.5 map images collared and stitched together using called a program called ERMapper, which we don’t have anymore.
  3. Insert a new map frame and load the quadrangle jpg image. Check the coordinate system of the map and image, again. Make sure they’re the same.
  4. Import the map tiles shapefile to your database and then add it to the map.
  5. Under the “list by selection” tab in the table of contents panel (at the top), uncheck you “map_tiles” layer.

    This action keeps you from accidentally selecting it and editing or moving it. This is a grid that we each will “snap” to during the digitizing, so it can’t move! (well… it can, but you shouldn’t).
  6. Right click on the “map_tiles” layer and check “label features” and tile numbers will be displayed. Zoom in to an area that is centered on and a little bigger than the tile assigned to you.
  7. Bring in the .jpg scan of your part of the geologic map.
    It might warn you that it can’t be projected, but we know that, because it is just a scan without coordinates so far.
  8. Select the geol map scan in the table of contents and from the imagery tab, select the start georeferencing tools button (note what it says in the help files about saving!).
  9. Perform steps 6 and 8-10 in the help website.
  10. Make “control point” pairs (one set of coordinates will be in pixels on the image, and the second set of the pair will be the UTM coordinates where you want it to be) by “add control points” button then click first on the image point of comparison (the green cross), then second on the topo map point (red cross) of comparison (shown below as the same intersection, with geo map transparent over the topo map). You can zoom and turn layers on or off between the two clicks if that helps. Make sure you can see same topo feature in both before you start.

    Move around to get points near the map corners and then fix the middle if necessary. Do not use the straight pencil lines on the scan for points; they do not exactly match the lines in the map_tiles layer. If you make a mistake, finish the collection of that data pair, open the table from the Georeferencing toolbar (“Control Point Table”) and delete the last point. DO NOT USE CTRL Z because this might do something like remove your scan and all the data with it. As you collect points, open the table and save the file of points to your homework folder. Do this often and especially when you are finished with the rectification. This is also a good piece of metadata, and you should note the RMSE for your metadata (alt+PrintScreen). RMSE is given in the units of the destination for the transformation. Do you know what the cell size is of the topo map? Your error should approach the cell size of that raster, but it won’t likely be less than the cell size.
    –Post a couple of snips of your control points with the geology transparent over the topo map.
    –And when you’ve got all you need, post one zoomed out showing them all.
    –Post a snip of your final Control Point Table
  11. When you’re satisfied, set the transformation to something that works well (you shouldn’t have to “warp” it much. It is a UTM-like map already). Save the data to your jpg image, then Save As New with a good name to your folder or to your geodatabase.  If it asks, in the export tool, choose the “Nearest Neighbor” but I would definitely use the lowest order of transformation (likely 1st order affine: rotate and scale only).
    Georeferencing or Geolocating. See the notes page here on “Geolocating images” if you have questions.

Digitizing the Geology

  1. Now it’s time to digitize the outlines of the geologic units. We will use these units of rock (this is what your attribute table should look like.
    Unit Geology Lithology
    1 Qal* Alluvium
    2 Ost Carbonate
    3 Cco Carbonate
    4 Ce Carbonate
    5 Cwbs Shale
    6 Ca Sandstone
    7 Ch,Chs Sandstone
    8 Cu Phyllite
    9 Yb, Yl, Cz crystalline
    *include the river (digitize right through it)
    and ignore terrace deposits “td” (figure out what is “under” them)
  2. Did you remember to turn off the “selection” tab option for the “map_tiles” layer? Don’t edit that layer by accident; many have in the past to ill effect.
  3. Create a new polygon feature class in your project geodatabase with the following info.
    1. Name your new map geo##_username where ## is your map tile number above (01, 02, etc) and “username” is your WLU userid.
    2. Note: Make sure to set the coordinate system! It should be exactly the same as the map view and data in the open project. If you digitize from a projected data layer (e.g., a NAD-83 data layer projected into a NAD-27 data frame) you rely on the “on-the-fly” reprojection routines that may induce errors of several hundred meters at some locations on the map, even though the location distance between the two datums is just 20-30 meters.
    3. Set up the fields in your new Feature Class.
      1. Unit (short integer)
      2. optional – Geology (text)
      3. optional – Lithology (text)
        you can join the optional ones later using the Geo_Units_Table.csv file in the digitize folder.
  4. Select the new feature class and go to the Editing tab.
  5. Set the snapping environment as directed here (http://pro.arcgis.com/en/pro-app/help/editing/enable-snapping.htm) making sure that the point, end, vertex and edge “agents” are on. You can mess with other options (distance, color, etc) as you like.
  6. In the table of contents, “List by Snapping” pane, make sure that the map tiles layer in addition to your geology polygon is enabled for snapping.
  7. Digitizing
    1. NOTE!! Start in one corner of your map with a regular polygon, and then work outward by appending new polygons to the previous ones (Auto Complete Polygon).  See Complete Adjoining Polygons in help.
      — Post a snip of your first polygon
    2. Do not digitize complete polygons next to each other! They need to “share” the line between them.
      –Post a snip of one of your appended polygons.
    3. Please make sure your digitizing snaps to the map_tile layer, so that when we merge the data, no “slivers” will be formed. Note that the “map_tile” lines may not exactly match the pencil lines on your scan of the geology map. The map_tiles lines are where you should start and stop your map digitizing.
    4. Save often!
  8. Once all of the digitized areas are available, each student will make a single Geology map that covers the entire area, selecting only one of any tile that is repeated.
    1. Post a snip of your digitized geology map overlain on the scanned geology map.
    2. If you are working in a campus lab, copy your geology feature class to the geo_maps_2022.gdb  geodatabase in the folder

      1. finish all your editing (and saving),
      2. Right-click on the “Folders” in the catalog pane
      3. “Add a folder connection”
      4. Migrate to the “Q:\Courses\GEOL260\ folder, and select ”completed_geo_maps” (do not open it)
      5. Click “OK”
      6. You should now be able to open your geodatabase in the Catalog Pane, right click on your layer and choose “copy”
      7. Right-click on the “completed_geo_maps” geodatabase in that newly added folder and choose Paste.
      8. Please do not delete or remove anything from that folder, if you need to recopy it, please just add a new one.
    3. Once they are all complete (I will alert everyone by email), copy this Geodatabase to your own project folder (or copy the data out of it into yours). Examine the other students’ efforts and import 10 geology panels into your own project geodatabase
      –Post a snip of the individual map tiles (similar to below)
    4. Merge them using the Merge (data management) Geoprocessing tool (do you know what that tool does? read a bit if you don’t)
      –Post a snip of the merged map tiles (similar to below)
    5. Dissolve the boundaries between matching units using the Dissolve (data management not coverage) Geoprocessing Tool. Use “Unit” as the dissolve field.

      –Post a snip of the dissolved map tiles (similar to above) 
    6. Examine it for errors (You can see “fixing topology errors” in the help menu) and use these in the “accuracy” discussion.
      — Find, zoom in to and post a couple of snips of errors that will need fixing (holes, overlaps, etc)
    7. Post your assessment of the accuracy and precision of this geology layer based on our construction process. Accuracy and precision are two different things; ask if you don’t know.
      1. Precision should be quantitative, indicating distances in meters.
      2. How might you assess this for the two processes (georeferencing and digitizing)?
      3. What kinds of errors are possible (spatial? categorical? topological?)?
      4. How do you locate errors?
    8. 5 points of this week’s exercise are for how well you georeferenced and digitized your map (error free and in the right place/format)
    9. Want  10 points extra credit? Fix all the errors in your merged and dissolved map (A or B) using the topology tools and attribute table as necessary. (You’ll likely have to read up about topology tools, see lab template for details)