Each GCD project contains a survey library. Conceptually, the GCD considers each DEM to be a separate "survey". and the Survey Library is where you load all your topographic surveys for use in change detection analyses. The survey library is intended to house all your surveys through time for a given study site. Different study sites on the same river, for example, will have non-overlapping spatial extents and should be placed in separate *.gcd Projects.



The Survey Library has three commands:

 Remove Existing Survey

Surveys successfully added in the survey library will be available for subsequent change detection analysis.

It is important to realize that the main reason the survey library exists is to help you organize your data for change detection analyses and enforce some of the best practices (e.g. orthogonality) required to do subsequent analyses. As such, the survey library makes a copy of all data loaded into the project, and organizes this in the input folder of the project.

You should create each new project in a separate folder. Do not store multiple GCD projects in the same folder.

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DEM Survey Properties Dialog

The DEM Survey Properties dialog has three primary functions:

1. Load topographic surveys into your *.gcd Project
2. Associate various surfaces and datasets with those surveys (e.g. inputs into an FIS)
3. Produces various elevation uncertainty models (error surfaces) of your survey

These three functions are organized onto three different tabs of the DEM Survey Properties dialog  (DEM Survey tab, Associated Surfaces tab, and the Error Calculations tab):

This video shows how to add a survey to the survey library. When adding mask polygon layers to the survey library, note that these feature classes must be flat and not contain and "Z" elevation coordinates.

Note, if your rasters do not meet the GCD's dimensional divisibility and orthognality criteria, you will be prompted to fix them.

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Associated Surfaces Tab

Associated surfaces are rasters that represent additional information about your survey. They are used in the calculation of error rasters. Common associated surfaces are slope or point density. Each associated surface is represented as a single raster dataset.

This video shows you how to add associated surfaces to your survey.

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Error Calculations Tab

Each survey can have multiple error calculation rasters. There are four types of error rasters in the GCD:

• Uniform error
• Spatial variable error based on a fuzzy inference system
• Hybrid error mixing either uniform error values or uniform error values with fuzzy inference system values.
• User defined error rasters.

This video shows how you can create a uniform error raster:

This video shows how you can load a user defined error raster:

This video shows how you can calculate an FIS error raster using a pre-loaded FIS and associated surfaces as inputs:

For more information on fuzzy inference systems and how they can be used to calculate elevation uncertainty surfaces, see here.