GIS In Archaeology

Lab Exercise 6

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: Identify Raster Sets
Start ArcCatalog and open the folder E:\GIS_In_Archaeology\Data\Mexico\Rasters. There are 4 Raster data sets listed: Airphoto, DEM, Landsat and Quadsheet. You can recognize that these are Raster data sets by the grid icon next to each data set name.

Step 3: Examine Raster Data Files
Double click on the My Computer icon and then navigate to the computer folder E:\GIS_In_Archaeology\Data\Mexico\Rasters. Note: alternately you can use Windows Explore if you’re more comfortable with that. While there are 4 raster data sets listed in ArcCatalog, there are 8 files (each of the data sets listed has 2 files with the same name).

Step 4: Open Raster Data Header File
For each data set name (Airphoto, DEM, Landsat, Quadsheet) there is one large sized file (Airphoto.tif, DEM.bil, Landsat.bsq, Quadsheet.tif), and one small sized file (Airphoto.tfw, DEM.hdr, Landsat.hdr, Quadsheet.tfw).
The general format of many Raster data sets is that there is one large file that contains the actual data values for the data set and another “header” file that contains information on the spatial placement and extent of the data set.
Use Notepad to open the DEM.HDR file and examine it’s content. In windows right-click on the DEM.HDR file and select Open from the pop-up menu. Windows will tell you that it cannot open the file. Choose the option “Select the program from a list”. From the Open With dialog window select the program Notepad from the list.

Step 5: Header File Contents
The header file for the DEM raster data set tells us that the data has 1816 rows and 1283 columns of data. The upper left hand corner of the data set is at x-coordinate 430020 and y-coordinate 2154960. It also tells us that each raster cell in the data set measures 60 units in the x-dimension and 60 units in the y-dimension.

Step 6: Raster Data Set Properties
Return to ArcCatalog right-click on the DEM data set and select Properties from the pop-up menu. Notice that under the section for Raster Information the properties for Columns and Rows as well as Cellsize (X,Y) are the same as the values in the header file. Scroll down the properties list to the entry for Spatial Reference. Currently the data set has no spatial reference information associated with it.

Step 7: Assign Spatial Reference Information
To assign the spatial reference information click the Edit button next to the entry for Spatial Reference. This will bring up the Spatial Reference Properties window we’ve seen in previous exercises. Navigate this window to assign the data’s spatial reference to be UTM, NAD 1927, Zone 14N.


Step 8: Calculate Statistics for the Data Set
ArcGIS can do a better job manipulating the raster data set once it knows the distribution of the data values in the data set. Go down to the Statistics entry in the properties window click the Options button and choose the Build Statistics entry. On the Calculate Statistics dialog window click OK. Once the statistics are successfully calculated close the Calculate Statistics dialog window.

There seems to be a slight glitch in ArcGIS, when it returns to the Property window after closing the Calculate Statistics dialog window the Property window no longer references the DEM data set. Close the property window, then re-open the Property window for the DEM data set.
Scrolling down the property window the data set now contains information on the spatial reference of the data set, as well as information on the data distribution of the data set. Under the Statistics entry we can see that the DEM data set has a minimum elevation of 785 meters, a maximum of 3975, and a mean elevation of 2043 meters.

Step 9: Update Spatial References and Statistics of Raster Data Sets
Following the procedure above assign the spatial references and statistics for each of the other data sets. The spatial reference for the Airphoto data set is UTM, NAD 1983, Zone 14N; and for the Landsat and Quadsheet data sets is UTM, NAD 1927, Zone 14N.

Step 10: View Data Sets in ArcMap
Launch a new version of ArcMap and begin a new map document. Following the procedures in previous exercises add the raster data sets (Airphoto, DEM, Landsat, and Quadsheet) to the map document as well as the vector data set States.
Modify the symbology for the States layer to be hollow with red outlines.
Modify the layer order in the Table of Contents so that States in on top, followed by Airphoto, Quadsheet, Landsat and DEM.
Zoom in to the area covered by the raster data sets.

In ArcMap use the zoom and pan tools as well as turning layers on and off to examine the various data layers.

Step 11: Modifying Raster Symbology
Turn off the visibility for the Airphoto, Quadsheet and Landsat layers so just the States and DEM layers are visible. Currently the color scheme for the DEM layer uses a grascale classification scheme (darker shades are lower elevations, lighter shades are higher elevations). This classification scheme does not provide a very good contrast to adequately visualize the topographic variation of the area.


Right click on the DEM layer and select Properties from the pop-up menu to bring up the properties for the layer. Click the symbology tab of the Layer properties to show the current layer symbology. Notice that the layer is currently using a Stretched classification scheme, using a black-to-white Color Ramp, the High value of 3975 is associated with the white end of the color ramp and the low value of 785 is associated with the black end of the color ramp. Also notice that the data is currently using a Standard Deviation Stretch based on 2 standard deviations.

We will now use a different color ramp which will do a somewhat better job at displaying the topographic variation. Click the drop-down arrow to the left of the black-to-white color ramp, a variety of different color ramps will appear. Scroll down various color ramps until you find one that goes from a light blue to green to yellow, red, brown and finally white. Select this color ramp and then click the OK button. Notice how it is now possible to easily determine that in general the southern end of the region is characterized by generally lower elevations than the northern half. Also note that there is a higher elevation mountain range splitting the northern end of the map.

Feel free to play with other classification schemes and settings to see how they affect the display of elevations.

Step 12: Load ArcGIS Spatial Analyst
In ArcGIS there are a number of tools available to manipulate raster data sets. These tools are part of the Spatial Analyst Extension toolbox and are not automatically available for use. To enable the Spatial Analyst extension go to the Tools menu and select the Extensions selection. This will bring up the Extensions dialog window. On the extensions dialog window make sure the the box next to Spatial Analyst is checked and the Close the window.

Next we need to make the Spatial Analyst toolbox. From the View menu select Toolbars make sure the entry for Spatial Analyst is checked. This will make the Spatial Analyst toolbar visible. Drag the Spatial Analyst Toolbox to a position that is convenient for you.

Step 13: Create a New Temporary Working Folder for Spatial Analyst
During many operations the Spatial Analyst tools will create temporary files in a default windows working directory. This is not necessarily convenient since it can be hard to find these layers should we wish to reuse, or remove, them later.
Go to ArcCatalog and right-click on the Lab_Data folder. Select the New Folder option from the pop-up menu.

A new folder called New Folder is added to the list

Right click on the New Folder entry and select Rename from the pop-up menu. Change the name of the folder to be Temp

Step 14: Set Spatial Analyst to Use the New Temporary Working Folder
Return to ArcMap.
Click the down arrow next to the Spatial Analyst entry in the Spatial Analyst Toolbox. On the Options dialog window click the Browse icon next to the Working directory entry. Then from the Choose a working directory select the V:\Anth497G\Lab_Data\Temp folder. Click OK to save this as the working directory.

Step 15: Create a Contour Map
One way to make elevation maps more interpretable to humans is through the use of contour maps. Click the down arrow next to the Spatial Analyst entry in the Spatial Analyst Toolbox. Select Surface Analysis Contour from the drop down menu. Make sure the DEM.BIL is the selected Input surface, set the Contour interval to 100 and the Base Contour to 700 (this is because the lowest elevation in our data set is 785 meters).

A new entry with a name like ctour2 is added to the maps table of contents and a contour line layer has been added to the map.

Explore the new contour line layer and then remove it when you are done – we will come back to contour mapping in a future lab.
Step 16: Create a Hillshade Map
Many times it’s easier to perceive topography when it’s presented in a 3-D type of format. A simple form of 3-D representation we can use is a Hillshade map. Click the down arrow next to the Spatial Analyst entry in the Spatial Analyst Toolbox. Select Surface Analysis Hillshade from the drop down menu. Make sure the DEM.BIL is the selected Input surface, leave the rest of the default values the same.

A new layer called Hillshade of DEM.BIL is added to the table of contents and the map now displays the elevation data to show light and dark areas if the sun was shining on the area.

Try creating other hillshade graphics using other options on the Hillshade dialog window to understand how they work.
Step 17: Combining Raster Layers for Display
Both the color coding and the Hillshading of the DEM elevation layer have been helpful in interpreting the topography of the area. Modify the layer order in the Table of Contents by dragging the Hillshade of DEM layer below the DEM layer. For the moment the Hillshade layer will be hidden.
Right click on the DEM layer and bring up the Properties window. On the property window select the Display tab and modify the value in the Transparent box to be 50 %.

The map document now shows the color coded elevations with the hillshade map showing through. By combining these layers the topography is easier to understand.


Step 18: Deriving Slopes from the Elevation Map
One factor that can influence the locations that human populations choose to settle is the slope of the topography – if the slope is too great it can be difficult to build a settlement there.
Click the down arrow next to the Spatial Analyst entry in the Spatial Analyst Toolbox. Select Surface Analysis, Slope from the drop down menu. Make sure the DEM.BIL is the selected Input surface, leave the rest of the default values the same.

A layer for the slope is added to the table of contents and the map shows areas of low slope in green, high slope in red. This isn’t a very interpretable map from a human perspective but the GIS now contains the slope of each 60 meter cell in the region.


Step 19: Calculating Distance to Water variables
A second variable that can influence site locations is the availability of water. As a general rule the distance to a water source can often be an important factor.
Turn off the slope layer, then go to ArcCatalog and add the layer Rivers (in the Mexico folder) to the map document. The map now contains the major waterways in the State of Morelos in Mexico.
Click the down arrow next to the Spatial Analyst entry in the Spatial Analyst Toolbox. Select Distance, Straight Line from the drop down menu. Select Rivers as the Distance to: layer, set Maximum distance to be 10000 (once we get beyond 10 Km we’re just far from water), set the Output cell size to be 60 to match the cell size of the DEM layer.

The map contains a layer that measures the distance of cells from the nearest river. Currently on the map it looks like a series of concentric lines but that’s just because of how the data are symbolized into 1000 meter groups. You can verify this by using the identify tool with the Distance to Rivers layer to view some distance values.


Step 20: Make Temporary Data Sets Permanent
Currently the layers we created using the spatial analyst tool are all temporary data sets that are stored in the temporary folder we specified in step 14.
Make the Distance to Rivers layer permanent by right-clicking on the layer then selecting the Make Permanent option from the pop-up menu. Change the name of the layer to be WaterDist and save it in the V:\Anth497G\Lab_Data\Mexico\Rasters folder.

Repeat the above step for the Slope of DEM layer and save it in the Rasters folder with the name Slope.

Step 21: Combining Slope and Distance to Water
If both lower values for slope and lower values for distance to water can be considered as likely attractors of human settlement then it might make sense to combine these two values into a single summary value. Using Map Arithmetic, one operation we can use with Raster data sets is adding the two layers together.
Click the down arrow next to the Spatial Analyst entry in the Spatial Analyst Toolbox. Select Raster Calculator from the menu. In the Raster Calculator window double click on the entry for Distance to Rivers, then click the addition symbol , then the Slope of DEM entry. This will create the expression [Distance to Rivers] + [Slope of DEM.BIL] in the expression window. Once you have finalized this click the Evaluate button to perform the operation.

The map now contains a layer that is the addition of the values from the Slope and Distance to Water raster data sets.

Step 22: Analysis Question
If both Distance to Water and Slope can be considered to have influenced human settlement decisions why is the summary layer we just computed probably not a very good predictor of human settlement decisions?
How might we modify our approach to provide a more accurate analysis using these two variables?

Step 23: Save Map and Exit
Save the current map document in the Anth497G folder under the name Lab_06.mxd We will work with this map in the next lab class.