ModelBuilder to automatically calculate area of feature for use in field calculator

Question

A common task I have is to take a site area that encompasses several different properties and make a map showing how much of each property is within the site boundary. Then I calculate the proportion of each property compared to the site area. The properties are all single part polygons in a feature (white lines) and the site area is a single polygon (red line).

I made a ModelBuilder tool to automate this, but I can't figure out a good way to automatically calculate the total site area and then use that number in the field calculator to calculate the property proportion for each property.

The model uses the intersect tool to create a feature clipping each property to the site boundary and calculates the area of each property within the site boundary. This is where I need help. Currently, I have a model parameter to manually input the total site area. ModelBuilder uses this value to calculate the property proportion for each property with the equation below.

To lower risk of mistakes, I want to automatically sum the property areas calculated from the intersect tool and use that number as the total site area to calculate property proportion. This way I know that the property proportions will add up to 100% and I don't have to calculate the total site area myself to input manually.

I'm using ArcMap 10.8 and am not familiar with Pyhton (unfortunately).

Answer 1

You were very close to completing the model, you just needed to add a couple more tools to the model.

You need to be aware that the intersect transfers over the area of the polygons before the actual intersect, especially if your output is a shapefile, so typically you need to recompute the new areas created by the cutting up process of the intersect.

A better model would be this:

The Get Field Value tool is a model only tool and allows you to extract a value from the first row, in this case the area of the site (which is a shapefile of just 1 polygon).

The model then intersects the site and parcel data, deletes out any area fields, recomputes a new area for the polygon (which is now the result of the intersect), adds a proportion field and computes the proportion into it.

The field calculate uses inline variable substitution to create the correct calculate:

[POLY_AREA] / %Site Area%

%Site Area% is the output of the get field value tool.

As a side note I would recommend you DO NOT change the name of the tool, the text you see in the yellow box, as then you have no idea what the tool is. Changing the name of the inputs/outputs is OK.