gis

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.

Introduction

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).

Dissolving

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.

dissolve

Calculating Area

g13

Calculate Areas as a Proportion of the total

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:

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.

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