GenerateGCPsFromTiePoints Task

This task generates two sets of ground control points (GCPs) from input tie points. The geographic locations of the GCPs are calculated from the first raster. The elevation values of the GCPs are calculated from the DEM raster.

You can use the resulting GCPs in ENVI applications such as RPC Orthorectification, Image-to-Map Registration, DEM Extraction, Build RPCs, and Rigorous Orthorectification.

Example

This example uses sample images from the RPC Orthorectification tutorial. Tutorial files are available from our ENVI Tutorials web page. Click the RPC Ortho link to download the .zip file to your machine, then unzip the files. Update the file references in the example with the correct locations.

; Start the application

e = ENVI()

; Open input rasters

File1 = 'NAIPReferenceImage.dat'

File2 = 'OrbViewSubset.dat'

DEMFile = 'DEM.dat'

Raster1 = e.OpenRaster(File1)

Raster2 = e.OpenRaster(File2)

DEMRaster = e.OpenRaster(DEMFile)

; Automatically generate tie points

Task = ENVITask('GenerateTiePointsByCrossCorrelationWithOrthorectification')

Task.INPUT_RASTER1 = Raster1

Task.INPUT_RASTER2 = Raster2

Task.INPUT_DEM_RASTER = DEMRaster

Task.REQUESTED_NUMBER_OF_TIEPOINTS = 40

Task.Execute

; Filter the tie points

FilterTask = ENVITask('FilterTiePointsByGlobalTransformWithOrthorectification')

FilterTask.INPUT_TIEPOINTS = Task.OUTPUT_TIEPOINTS

FilterTask.INPUT_ORTHORECTIFIED_TIEPOINTS = Task.OUTPUT_ORTHORECTIFIED_TIEPOINTS

FilterTask.Execute

; Generate ground control points

GCPTask = ENVITask('GenerateGCPsFromTiePoints')

GCPTask.INPUT_DEM_RASTER = DEMRaster

GCPTask.INPUT_TIEPOINTS = FilterTask.OUTPUT_TIEPOINTS

GCPTask.Execute

The GCPs and tie points are written to the specified directory.

Syntax

Result = ENVITask('GenerateGCPsFromTiePoints')

Input parameters (Set, Get): DEM_IS_HEIGHT_ABOVE_ELLIPSOID, INPUT_DEM_RASTER, INPUT_TIEPOINTS, OUTPUT_GCPS1_URI, OUTPUT_GCPS2_URI

Output parameters (Get only): OUTPUT_GCPS1, OUTPUT_GCPS2

Parameters marked as "Set" are those that you can set to specific values. You can also retrieve their current values any time. Parameters marked as "Get" are those whose values you can retrieve but not set.

Input Parameters

DEM_IS_HEIGHT_ABOVE_ELLIPSOID (optional)

Set this parameter to true if the DEM is already expressed as the height above the ellipsoid and no geoid correction is required. Most DEM data contain orthometric heights, which are elevations above mean sea level (for example, GMTED2010 or National Elevation Dataset, NED). In these cases, keep the default selection of false.

INPUT_DEM_RASTER (required)

Specify a digital elevation model (DEM) raster. The elevation values of GCPs are calculated using the DEM raster. If you do not have a DEM file readily available, you can use the Global Multi-resolution Terrain Elevation Data 2010 (GMTED2010) DEM named GMTED2010.jp2 that is provided under the ENVI installation folder. The GMTED2010 dataset has a mean resolution of 30 arc seconds. However, for best results, you should use a DEM raster with a higher resolution than GMTED2010. Many DEM products are available from the U.S. Geological Survey National Map and Download Platform or EarthExplorer.