Apply map algebra

Apply map algebra to an elevation raster to floor, mask, and categorize the elevation values into discrete integer-based categories.

Use case

Categorizing raster data, such as elevation values, into distinct categories is a common spatial analysis workflow. This often involves applying threshold‑based logic or algebraic expressions to transform continuous numeric fields into discrete, integer‑based categories suitable for downstream analytical or computational operations. These operations can be specified and applied using map algebra.

How to use the sample

When the sample opens, it displays the source elevation raster. Tap the Categorize button to generate a raster with three distinct ice age related geomorphological categories (raised shore line areas in blue, ice free high ground in brown and areas covered by ice in teal). After processing completes, switch between the map algebra results raster and the original elevation raster.

How it works

1. Create a ContinuousField from a raster file.
2. Create a ContinuousFieldFunction from the continuous field and mask values below sea level.
3. Round elevation values down to the lowest 10-meter interval with map algebra operators ((continuousFieldFunction / 10).floor() * 10), and then convert the result to a DiscreteFieldFunction with .toDiscreteFieldFunction.
4. Create BooleanFieldFunctions for each category by defining a range with map algebra operators such as isGreaterThanOrEqualTo, logicalAnd, and isLessThan.
5. Create a new DiscreteField by chaining replaceIf operations into discrete category values and evaluating result with evaluate.
6. Export the discrete field to files with exportToFiles and create a Raster with the result. Use it to create a RasterLayer.
7. Apply a ColormapRenderer to the raster and display it in the map view.

Relevant API

• BooleanFieldFunction
• Colormap
• ColormapRenderer
• ColorRamp
• ContinuousField
• ContinuousFieldFunction
• DiscreteField
• DiscreteFieldFunction
• Raster
• RasterLayer
• StretchRenderer

About the data

The sample uses a 10m resolution digital terrain elevation raster of the Isle of Arran, Scotland (Data Copyright Scottish Government and SEPA (2014)).

Additional information

This sample requires an ArcGIS Maps SDK Analysis extension license key. Without this license, the map algebra analysis will fail at runtime.

Tags

elevation, map algebra, raster, spatial analysis, terrain