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Spatial Analyst—Introduction

Spatial Analyst—Introduction

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Introduction (PDF; 525 KB)

ArcGIS 9.0 Introduction to Spatial Analyst for Initial and Update/Maintenance Soil Surveys

Updated November 2005

Goals

  1. To be able to use datasets derived from elevation data for soil survey applications.
  2. To provide soil scientists with tools to help view, describe, and analyze the relationship between landform/landform positions and the components found on those positions.

Overview of Spatial Analyst

For soil survey activities, Spatial Analyst provides a mechanism to analyze the relationship between DEMs (rasters) and Soil Polygons (vectors).

Rasters are cells that are arranged in rows and columns.

  • DEMs are rasters that have a specific elevation value in each cell (cell sizes are typically 10 or 30 meters).
  • DOQs are rasters that have a specific pixel value in each cell (cell sizes are typically 1 or 2 meters).

Vectors represent geographic data, such as points, lines, and polygons.

The differences between rasters and vectors:
  • Rasters have continuous data based on cells without defined boundaries. Each cell can have a different value.
  • Vectors have defined boundaries and are uniform within the boundary.

Understanding DEMs

How are DEMs created?
  • Hypsography (developed from contour lines); most DEMs are created by this method.
    • (Note: Hypsography method has a contour line bias that can create unintended results.)
  • Photogrammetry
  • Survey points from survey-grade GPS
  • Satellite Imagery

DEM Limitations

DEMs are a useful and powerful tool, but you need to understand their limitations.

  • The “M” in DEM stands for Model.
  • DEM Limitations
    • Random errors (from misplaced contour lines)
    • Contour line bias (limitation of Hypsography)
    • Scale error (landform features are finer than the resolution of a DEM; e.g., short steep slopes

Know the limits of your data.
Check metadata about scale and resolution.
Make sure 10-meter DEM data is not resampled 30-meter data.