15. Raster Analysis¶

Goal: Run the most common raster operations — terrain analysis, vegetation indices, reclassification, and multi-criteria overlay.

You need Spatial Analyst¶

Most raster tools live in the Spatial Analyst extension. Confirm it's licensed: Project → Licensing → Spatial Analyst = checked.

Raster Functions vs Geoprocessing tools¶

ArcGIS Pro has two ways to run raster operations:

Way	When
Raster Functions (Imagery ribbon → Raster Functions)	Lazy / on-the-fly. Fast, doesn't write to disk until exported.
Geoprocessing tools (Spatial Analyst toolbox)	Writes a permanent output raster.

Use Raster Functions to explore. Use Geoprocessing to save results for analysis.

Terrain analysis¶

From a DEM:

Tool	Output
Slope	Steepness in degrees or percent
Aspect	Direction the slope faces (0–360°)
Hillshade	Shaded relief — pure cartography
Curvature	Concave / convex / flat
Contour	Vector contour lines
Viewshed	What's visible from a point

Quick recipe: hillshade for a basemap¶

Spatial Analyst → Hillshade on the DEM.
Symbology: stretch to display.
Set the layer transparency to 50%.
Place over a colored elevation layer.

Map algebra & Raster Calculator¶

The Raster Calculator lets you do math across rasters cell-by-cell.

# NDVI from Landsat 8
("Band5_NIR" - "Band4_Red") / ("Band5_NIR" + "Band4_Red")

# Areas above 1000 m AND south-facing
("DEM" > 1000) & ( ("Aspect" >= 135) & ("Aspect" <= 225) )

Result is a raster: 1 where the condition is true, 0 where false.

Reclassify¶

Convert a continuous raster into discrete categories.

Tool: Reclassify (Spatial Analyst).

Old value range	New value
0 – 1000	1 (low)
1000 – 2000	2 (medium)
2000 – 5000	3 (high)
NoData	NoData

Reclassify is the foundation of suitability analysis — you're putting all criteria on a common scale (1–10 or similar).

Zonal statistics¶

"What's the average elevation inside each watershed polygon?"

Tool: Zonal Statistics as Table (Spatial Analyst).

Zone	Value
Zone layer	A polygon layer (or another categorical raster)
Value layer	The continuous raster to summarize
Statistic	MEAN, SUM, MIN, MAX, STD, MAJORITY…

Output is a table linkable back to the zone layer.

Weighted overlay (suitability)¶

"Find the best site, given multiple criteria"

flowchart LR
    C1[Slope reclass<br/>1–10] --> WO((Weighted<br/>Overlay))
    C2[Distance to road<br/>reclass 1–10] --> WO
    C3[Land use<br/>reclass 1–10] --> WO
    C4[Floodplain<br/>reclass 1–10] --> WO
    WO --> Out[Suitability raster<br/>1 = poor, 10 = ideal]

    classDef in fill:#eef2ff,stroke:#4338ca,color:#312e81
    class C1,C2,C3,C4 in
    classDef tool fill:#fef3c7,stroke:#f59e0b,color:#92400e
    class WO tool
    classDef out fill:#dcfce7,stroke:#10b981,color:#065f46
    class Out out

Steps:

Reclassify each criterion to a 1–10 scale.
Run Weighted Overlay: assign each criterion a weight summing to 100%.
Output is a single suitability raster.

→ Project: Site Suitability Analysis.

Common operations cheat sheet¶

Goal	Tool
Steepness map	Slope
Direction-facing analysis	Aspect
Pretty terrain visualization	Hillshade + colored elevation
Vegetation health	NDVI (raster calculator or Raster Function)
Land cover classification	Image Classification Wizard
Flow / watersheds	Flow Direction → Flow Accumulation → Watershed
Convert raster to polygon	Raster to Polygon
Convert polygon to raster	Polygon to Raster
Mask to a boundary	Extract by Mask

Pitfalls¶

Cell size mismatch

Map algebra requires inputs to have the same cell size and snap. If they differ, ArcGIS Pro will resample on the fly — usually fine, but check your Environments → Cell Size and Snap Raster for repeatable results.

NoData propagates

Any cell where one input is NoData will produce NoData in the output. Set NoData carefully or fill with Con() first.

Practice¶

Solar suitability

DEM → Slope, Aspect.
Reclassify Slope: gentle slopes get high score.
Reclassify Aspect: south-facing (135–225°) gets high score.
Weighted Overlay (60% slope, 40% aspect).
Symbolize → identify cells with score ≥ 8.

→ Next: Layout Design.