GIS In Archaeology

Lab Exercise 4

Step 1: Load Exercise Data
Verify that all data have been copied to a folder called GIS_In_Archaeology on your working drive. For this handout the folder has been created on the E-drive of the system computer.

Step 2: Create a new map document
Start ArcMap and at the opening dialog box select “A new empty map” to open up a blank map document. Add the entries for Countries and Meso_Sites to the map document to create a simple map of the distribution of some major sites in Mesoamerica.

Step 3: Examine the distribution of Classic Maya sites
Go to the main menu and choose Select By Attributes from the Selection menu

This will bring up the Select by Attributes window. Make sure that the Layer selected is Meso_Sites. We want to set up a query to select all Maya sites that were occupied during the Classic period. In the attribute table for Meso_Sites there is a field called Culture which stores the cultural affiliation of the site, and a field called Occ_Cl which will be marked with a “Y” if the site was occupied during the Classic period. So what we want to identify are all sites with a value of Maya in the Culture field and a value of Y in the Occ_Cl field.
To create this query first double-click on the entry for “Culture” in the Fields list (the entry for Fields in now transferred down into the large query window. Next click the button equal sign button and then choose the entry for ‘Maya’ from the Unique values list. The steps outlined here should have produced the result you see below.

If you click the Apply button now you should see that all the Maya sites become highlighted on the map. However, we only want to see the Maya sites which were occupied during the Classic period. To further restrict the selection go back to the Select By Attributes window.
To further restrict the selection first click on the And button then select “Occ_Cl” from the Fields list, click the equal sign button and then choose the entry for ‘Y’ from the Unique values list. The Select By Attributes dialog box should like this:

Click the Apply button and you should now have a map that looks similar to:

Step 4: Create Classic Maya Selection Layer
If you want to verify that you performed the steps properly open up the attribute table for Meso_Sites and you should see that 38 of the sites are presently selected. Zoom in on the Maya
area and you will see that most, but not all, of the sites in the region are Classic Maya sites. We will now create a subset of just the classic Maya sites.
Right click on the entry for Meso_Sites in the table of contents, then choose Selection and Create Layer from Selected Features from the pop-up menu.

Look in the table of contents and see that there is a new entry added for Meso_Sites selection which contains only Classic Maya sites. Turn off the layer for Meso_Sites and the map will only show the classic Maya sites. Rename the Meso_Sites selection layer to be Classic Maya Sites.

Step 5: Maya Site Territories
If you look at the map you see a fairly dense packing of sites in the area of northern Guatemala, near the border with Belize. The site points represent the centers of larger Maya sites that date to the classic period. We might also want to map in the land area which we think each of these individual sites might have controlled. One way that archaeologists have done this in the past is through the use of Thiessen polygons which delimit the boundaries of a site’s territory as falling halfway between the site and it’s nearest neighbors.
In ArcCatalog there is a raster graphic called Territories that uses Thiessen polygons to conceptualize the site territory of a number of the sites in this northern Guatemala area. Add the Territories graphic to your map. Zoom in on the area where these sites are located and turn on the labels for the sites to see the site names.

The map we are now looking at shows hypothetical boundaries between these sites but the site boundaries are simply a graphic and would not really be useful in a GIS analysis. We will create a GIS polygon shapefile that we can draw these sites boundaries in to.

Step 6: Create a New Site Territory Shapefile
The first step in creating a polygon shapefile of site areas is to create a blank polygon coverage. If you don’t already have it open, open up ArcCatalog. Select the entry for Lab_Data under V:\Anth497G. From the File menu, select New, then Shapefile

This brings up the Create New Shapefile dialog box. Change the Name entry from New_Shapefile to Territory and Feature Type from Point to Polygon

Looking at the dialog box as it currently stands you can see that the data set does not have any Spatial Reference data assigned to it (currently the description is Unknown Coordinate System). We will assign the data to be in North America Albers Equal Area Conic projection.
From the Create New Shapefile dialog box click the Edit button to edit the spatial reference information. This brings up the Spatial Reference Properties window that should look familiar from the last lab. Click the Select button to bring up the Browse for Coordinate System window.


In the browse for Coordinate System window select Projected Coordinate Systems, then Continental, then North America, and finally select North America Albers Equal Area Conic.prj.

Click the Add button to close the Browse for Coordinate System window and the OK button to close the Spatial Reference Properties window. In the Create New Shapefile window it now indicates that the N.A. Albers Equal Area Conic projection has been selected.

Click the OK button to add a new Territory.shp shapefile in ArcCatalog

Step 7: Add Site Territory to ArcMap
Return to ArcMap and add the Territory.shp shapefile to the map document, click OK if you receive a warning that the new layer has a different geographic coordinate system. The table of contents now has an entry for Territory but the map document has not changed any because there is no data in the Territory layer yet.

Step 8: Add Fields to the Attribute Table
Right-click on the entry for Territory and open up the Attribute table. Currently there are no rows in the table because there aren’t any features in the layer yet. The only fields available are FID, Shape, and Id

We want to add a field to the attribute table where we can enter the site center which the territory is associated with. To add a field to the table click the Options button then select Add Field from the pop-up menu.
On the Add Field window change the Name: to be Capitol (we will put the name of the main site here) and change the Type from Short Integer to Text.

Click OK to return to the attribute table and see that a new field called Capitol has been added to the table.

Step 9: Creating Site Territory Features
We are now ready to draw new site territories on the map. To modify the features in a layer we need to use the Editor tools. To add the Editor Toolbar, select View from the main menu, then select Toolbars, and Editor.

The Editor toolbar should now be added to your map document



The editor toolbar contains the following items
- Editor tool, contains the main editor menu options
- Edit tool for modifying feature shapes
- sketch tool for drawing and modifying features
- task bar for specifying the operation you want to carry out
- Target bar to specify the layer you are working with
- split tool for splitting features
- Rotate tool
- Attribute tool
- Sketch properties tool

Click the Editor tool then select Start Editing to begin editing the layer

Make sure that Create New Feature is the selected Task, and modify the Target to be Territory so that we will be drawing new features in the Territory layer.

Select the Sketch tool , your cursor should now turn in to a crosshairs icon.

The region around Tikal is looks similar to a triangle, but with 4 obvious intersection points. Draw the polygon by starting at one intersection then proceeding in a clockwise direction, and double-clicking on the 4th point to finish the polygon. If you make a mistake during drawing you can use the Undo Create tool to remove the last vertex you drew. If all goes horribly wrong just double-click to complete the polygon and then hit the Delete key to erase the bad polygon.

Next open up the attribute table for the Territory layer, there is now a single row in the attribute table which corresponds to the polygon you just drew. Click inside the Capitol cell and type in the name of the capitol site, Tikal.

Repeat the above process to draw the territory area around the site of El Naranjo.

Step 10: Setting Drawing Snapping Properties
We now need to draw the polygon for the territory around the site of Nakum. This territory lies between, and directly abuts, the territories for Tikal and El Naranjo. Obviously we want to draw this new polygon so that it precisely shares the boundary with the Tikal and El Naranjo territories. In a topological data set it would be possible to reuse the boundaries of the polygons that we’ve already drawn. However, the Territory layer is a shapefile dataset and recall from last class that shapefiles are a non-topological data set. We will need to draw the entire boundary for Nakum.
If we zoomed in close we could draw the polygon by hand so that it would come pretty close to sharing the boundaries with Tikal and El Naranjo. However, no matter how hard you try, it will never be possible to do that by manual means alone – there will always be at least a slight discrepancy. Fortunately, ArcGIS provides us with tools that can help us.


Click on the Editor menu and select the Snapping from the pop-up menu.

This will open up the Snapping environment window. When snapping options are set it means that when we draw an object, if the cursor is sufficiently close to a point that we have set up as a snapping point, the cursor will automatically be drawn exactly to that point. In this case we want to make sure that any new points can “snap” exactly onto other vertices in the Territory layer. Put a checkmark in the box for Territory Vertex.

In addition to setting what types of object to snap to we also need to set the snapping tolerance. Using snapping tolerance we can specify the threshold to use for snapping. Click on the Editor menu and select the Options from the pop-up menu.

The Editing Options window is now displayed. Select the General tab and in the Snapping tolerance box change the value to be 0.05 and Click OK.

Using these settings if we try to draw a new point when the cursor is within 0.05 map units of an existing vertex on the Territory layer then the new point will automatically snap to exactly that point.

Step 11: Drawing with Object Snapping
We now are going to draw the boundary around Nakum so that it directly overlaps the boundaries for Tikal and El Naranjo. To facilitate drawing zoom in on the territory around Nakum.
Again select the Sketch tool to being drawing a new polygon. Move the cursor around on screen to begin drawing the new polygon. Notice that whenever the cursor is within a certain distance from any of the existing corners of the Tikal or El Naranjo polygon, the cursor snaps on to the existing point.
Now draw the polygon in the same way that you drew the previous polygons but make sure that it snaps on to the 4 points that are already part of the previously drawn territories, and update the attribute table’s capitol field.

Repeat the above process to draw the other 6 territorial boundaries around the rest of the sites in this local area. Experiment with the snapping and distance options to view their effect.

Step 12: Save Current Map
When you have completed drawing all the polygons go to the Editor tool and select the Save Edits and Stop Editing options to save the changes you made to the Territory layer. Finally save the current map document as Lab_04.mxd and exit ArcMap.