Perhaps it would be nice to know what the area of each rock unit is and what percentage of the smaller map area it occupies. Here I will walk you through these simple calculations to illustrate the use of the geometry calculator for vector data sets.
Either Save your Task1 map as new Task2 document, or start a new document and add the UpdatedClippedGeoUnits
layer.
The first thing we want to do is determine the areas for each rock type.
If you open the attribute table for the UpdatedClippedGeoUnits
layer you will see that there is already an AREA field. There are three problems with this existing area field which came in the original shape file from the Geologic Survey. The first is that we don’t know the units for the values. Given that the projection is UTM, and default units are meters, it is likely that it was square meters, but we’d have to check. The second problem is that in Task 1 we clipped this data layer so all of the units intersecting the edge of the map will have inaccurate areas. Shapefile attributes (like the area field) don’t update when a new output file is created.
But in the bigger picture we have a bigger problem. We want to know which rock unit Types represent at least 5% of the total map area. If you look in the attribute table of the
UpdatedClippedGeoUnits
layer and sort the rock unit name field, you will see that there are multiple polygons for each rock type. To get a more accurate representation of rock type area, we need to combine these polygons into one record. To do this you can use the “dissolve” tool (Data Management).
The Dissolve tool allows us to ‘merge’ polygons based on attributes in a specific field (in this case “UnitName”). Don’t set the statistics to calculate SUM. Leave that, but make sure you leave the box checked to allow for multi-part polygons.
We will calculate new areas, this time in square kilometers.
UpdatedClippedGeoUnits
layer.AREA_SQ_K
field (you have to add this field to the dissolved table). Highlight the column by clicking on the AREA_SQ_K
header.AREA_SQ_K
header and bring up the Calculate Geometry dialog box:It doesn’t matter if you calculate in sq m or sq km as long as you keep track of your units.
Choose to calculate the area, using the data source’s coordinate system. Click
OK
.
You now have the area of the dissolved rock unit polygons for the extent of the data.
Now we want to calculate the percent of rock unit type relative to the map’s area.
To calculate percent of total area for each rock type, we first need to know the area of the whole map. You can either derive map area from the clip extent polygon (add a field to the attribute table and calculate area just like above) or sum the areas of the rock type polygons in the dissolved clipped rock type layer.
To do the latter:
Now you know the map’s area, Calculate the proportion each rock type’s area is of the total area:
Add another field “prop_tot” to the same attribute table
Right click the header > Field Calculator
Using the buttons, create an equation to divide the area field by the total map area.
Create a table for your website in which you report the rock names, areas with units, and proportion or percent of the total area (with clear units).
This task illustrated how you can do some simple analyses of spatial data.