unclear area increase through union input in model builder

04-29-2021 03:24 PM
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New Contributor

Hey everybody,

i got the following issue: The goal is to identify the size of landslides in 3 different zones, per district. Therefore i built a model via the modelbuilder. I calculated the size of each landslide beforehand using the "add geometry tool". The slides, districts, and each zone were my input for the first union (5 inputsi n total). From this Union I select each landslide with with the select tool and an expression like this: Process=Slide AND County=XY AND ZONE1=1 (that was my idea). Afterwards I used "statistics" and SUM in the attribute table for the total area of my landslides. I did the whole process for the total area, but afterwards during the evaluation of the results I realized there was a problem/ i made a mistake:

As long as the Input for the Union are ONLY the Counties and the slides, the slides area (only for each county) is quite realistic. In this case the expression e.g is: "Process=Slide AND County= XY" -> Sum: 9839ha (so far so good, that´s the whole amount of area for the slides located in every zone in the county). Now I DO NOT change the expression, only add some Input layers (the zones) for the Union. And this occurs: Area increases around 5 times, it seems like the areas of the zones are now also included in the calculation. Same happens if I use this expression "Process=Slide AND County=XY AND ZONE1=1" to identify the slides which are located in zone 1 the numbers get quite unrealistic.

Although I would like to have further however only the added area of all landslides which are within the respective zones. In the data frame nothing changes in the display in comparison to "only slides and county input", in the attribute table new objects occur.

How do I write the expression to avoid the  "double counting of the zones area"as it seems / make clear that I only want the slides which are located within the different zones (and get the area afterwards)? I use ArcMap 10.5.1. I hope someone understands what I´m trying to say. I´m grateful for every helping comment.




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MVP Frequent Contributor

I'm not entirely sure of your process but perhaps using Tabulate Intersection (Analysis)—ArcGIS Pro | Documentation then doing subsequent analysis on that table might be better.

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MVP Esteemed Contributor

A couple of things to check.

1. Is the Coordinate system same for all the feature classes?

2. Are you working with shapefiles? I recommend exporting shapefiles to File GDB Feature Classes, since the geometry attributes (Area, Perimeter) doesn't get recalculated automatically in shapefiles.

Then, use Dissolve (Data Management) tool. No Feature selection required.

- Use the Union Feature Class (in a File GDB) as input. And also save the output in a File GDB.

- Select PROCESS, COUNTY and ZONE1 as the Dissolve Fields

- Select Shape_Area as Statistics Field(s), and SUM as Statistics Type (You may choose to skip this step, since a Shape_Area field will be automatically be created in output summing up the area of each unique combination of values).

- Keep "Create Multipart Feature(s)" option checked.



Think Location
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