GIS 5265 Module 6: Digitizing and Editing Large Datasets

Module 6, spread out over two weeks, was quite a challenging and time-consuming task.  As in the previous module, we began by georeferencing a map image to an ArcMap basemap.  In this case, the image as a topographic map with a survey grid of the Oaxaca Valley, Mexico.  This was a frustratingly difficult task, as common points of reference between the image and basemap were difficult to locate.  By finding a couple of possible control points and updating the image's position as I went along, I was able to get a reasonably well-positioned image.  I then digitized by assigned three grid squares (first image above) and georeferenced the corresponding collection and land type maps to them.

The next task was to create new shapefiles for the collection units and land type.  While the land type shapefile was easy to complete (especially since I only have Type III land in my squares), digitizing the collection units was more tedious.  Square N9E8 in particular took some time to digitize properly.  The task was made more difficult by the presence of multiple units with separate components and the asymmetrical overlapping of units from difference periods.  As an example, in the northern portion of square N9E8 there is unit V-58, composed of four separate sections.  I thus created a multipart polygon to represent the unit as a single feature.  However, the westernmost circular component of V-58 also represents unit IIIA-47.  Therefore, I needed to create a new feature exactly corresponding to this component of V-58 (using the 'trace' editing tool) to represent unit IIIA-47.  I also had to interpret the labeling scheme for the dense cluster of units in the center of square N9E8.

Labeling the collection units of N9E8 was also problematic.  The complex combination of different time periods for the same components or portions of components effectively ruled out a color symbology.  Replicating the original labeling scheme was a possibility, but I decided to attempt a simpler scheme of indicator lines and arrows when necessary.  I used Illustrator to create the lines and arrows as well as to fine-tune label placement.

The map below represents an extra task graduate student completed.  We were given an Excel table with data on sites in the Oaxaca Valley.  By using the grid square column, we could isolate the sites located in our squares.  We were also to only use the sites dating to the IIIB Period.  I created a new table containing only the sites dating to the IIIB Period that were in my grids; this resulted in only two sites.  I then joined the table to my collection units layer in ArcMap based on unit name.  My initial join failed due to column names that were incompatible with ArcMap (mostly the presence of periods).  After editing the column names accordingly, the join went smoothly.  I then symbolized the two sites based on the conservative population estimate.  Unfortunately, having only two sites makes the map a bit uninteresting.  It also doesn't allow me to say much in way of interpretation.  Both the sites are relatively small and are located in the same land type (10% arable, not symbolized here).