Introduction to ArcGIS for Archaeologists
Practical 2 - Data analysis using GIS:
2. Performing attribute queries
Hopefully, you will now understand how visualisation is one powerful way of exploring your data using GIS. Another is through querying. The simplest form of querying is attribute based, and is very similar to constructing queries in any non-spatial databases you might have used. On the “Selection” menu, choose “Select By Attributes”. The following form should appear:
To begin, we shall try to select Roman records within the SMR layer. To do so, set the “Layer:” to the correct layer and ensure that the “Method:” is “Create a new selection”. The list below shows you the different attribute fields associated with that layer. Scroll down the list until you find “Period”, then double click on it. It should appear in the box at the bottom. This box is where we shall construct our query. Next click on the equals sign on the calculator. Then click on the “Get Unique Values” button. This will populate a list above the button with all of the possible periods recorded in the layer. Find “Roman” in the list and then double click on it. If you then click the “Verify” button, ArcGIS will tell you if your query should work. Click on “Apply” and look at the map. You should see that all of the Roman records in the SMR layer have been highlighted with a turquoise blob.
We shall now add another level of complexity to our query. Click on “And” in the calculator. Then find “Source” in the field list and double click on that. Then click on the equals sign again, then click on the “Get Unique Values” button. Double click on the “Excavation” value. The query should read as follows: "Period" = 'Roman' AND "Source" = 'Excavation'. Click on “Apply”. Now, you should see the selection change. It should show just the one record, next door to the school building. Thus, in just a few simple steps, we have discovered all of the Roman excavations in our region. Click on “OK” to close the form. Now, open the attribute table of the SMR layer (by right clicking on the layer in the layer list and selecting the appropriate command). If you scroll down the table, you will see that the selection has also been highlighted in there too.
If you go to the “Selection” menu and choose “Clear Selected Features”, the selection will be cleared. Experiment with the “Select by Attributes” tool and see if you can discover any interesting patterns in the data. For example, which SMR records relate to survey sites entered into the SMR after 1999? Pressing the “Clear” button will clear any queries currently constructed in the tool.