Homework 4 notes

Documenting a model

To document your model, edit its description, and add text to the following fields:

Be sure to click Save when you're done.

Shape_Area attribute

When a new polygon feature class is created, the Shape_Area attribute is created only when the feature class is written to permanent storage (i.e. a geodatabase or a shapefile). The Shape_Area attribute is not created for feature classes stored in the in_memory scratch workspace.

NoData: Reclassify

When using the Reclassify tool to create a mask, a nice shortcut is to just specify reclassifications for the input values you want to be 1, and force all other input values to map to NoData by checking the "Change missing values to NoData (optional) box. For example, to map slope values between 0 and 20 to 1, and all other values to NoData, you could do:

NoData: Zonal Statistics

When calculating zonal statistics, be sure to check the "Ignore NoData in calculations (optional)" box:

Otherwise, any watershed that contains any un-developable land (i.e., all of them) will have bogus statistics.

Polygon to Raster: Cell assignment type

Whenever you use the Polygon to Raster tool in Assignment 4, set "Cell assignment type (optional)" to MAXIMUM_AREA:

See "How Polygon to Raster works" in the ArcGIS online help for examples of how the various cell assignment types work .

Extract by Mask: When order matters

If you are using Extract by Mask to extract a range of values (not just 1s) from a raster, then that raster must the "Input raster". If a raster is used for the "mask data", then any non-NoData value behaves like a 1.

Say (for example) you have a raster of land values, and a mask of developable slopes:

If (and only if) both inputs are masks, then the order doesn't matter. (Pretend land values are either 1 or NoData and run the above example in you head to verify this.)

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