GCP File Formats

This topic describes the ground control point (GCP) formats that ENVI accepts. GCPs can be stored in ASCII files with .pts file extensions. Different geolocation tools in ENVI expect information in .pts files to be presented in specific formats. These formats are described below. In all cases, the .pts file headers must include a projection info string.

RPC Orthorectification Tool

The RPC Orthorectification workflow reads GCP shapefiles, as well as ASCII GCP files with .pts extensions. GCP files can be in any map projection, but the RPC Refinement panel will always display map coordinates in a Geographic WGS-84 projection.

In contrast to shapefiles, .pts files refer to the upper-left corner of the first image pixel as (1,1). If your image is a spatial subset that contains XSTART and YSTART offset values from the original image, the .pts file image coordinates include this offset. The RPC Orthorectification workflow always refers to the upper-left pixel as (0,0). Thus, it will subtract (1,1) from GCP image coordinates from a full image, and it will subtract the XSTART and YSTART values from the image coordinates of a subsetted image.

; ENVI Orthorectification Ground Control Points File
; projection info = {Geographic Lat/Lon, WGS-84, units=Degrees}
; Map (x,y,elev), Image (x,y)
-105.42543081    40.08088580    2000.000000    2372.000000    4134.000000
-105.37496362    40.12966926    1000.000000    4128.000000    1910.000000

Rigorous Orthorectification

; ENVI Rigorous Orthorectification GCP File
; FileName0=C:\03JUL17174528-M1BS-052157861010_01_P005.TIF
; FileName1=C:\03JUL17174525-M1BS-052157861010_01_P004.TIF
; FileName2=C:\05JUL04180115-M1BS-052157861010_01_P008.TIF
; FileName3=C:\05JUL04180112-M1BS-052157861010_01_P007.TIF
; projection info = {Geographic Lat/Lon, WGS-84, units=Degrees}
; ImageFile#, Map (x,y,elev), Image (x,y)
0       -105.358423         39.953100       2226.000000       4690.000000       3918.000000
2       -105.152796         39.910608       1757.000000       5565.000000       5813.000000

Build RPCs Tool

; ENVI Ground Control Points File
; projection info = {Geographic Lat/Lon, WGS-84, units=Degrees}
; Map (x,y,elev), Image (x,y)
-105.48775571    40.16771721    2000.000000     201.000000     200.000000

Exterior Orientation Points File

; ENVI Exterior Orientation Points File
; projection info = {Geographic Lat/Lon, WGS-84, units=Degrees}
; Map (x,y,elev), Image (x,y)
6.96035926    45.86210997    4001.000000    4269.000000    5237.000000
6.88871043    45.87880951    3842.000000    3686.000000    5165.000000

Stereo GCP File Created from the DEM Extraction Module

Note: The RPC Orthorectification workflow reads the first two values, ignores the next two values, and reads the last three values.

; ENVI DEM Extraction Stereo GCPs File
; projection info = {Geographic Lat/Lon, WGS-84, units=Degrees}
; Left (x,y), Right (x,y), Map (x,y,z)
5649.0000     7311.0000     6308.0000     8071.0000    -1.17410984e+002     4.77035482e+001     628.363000

Image-to-Map Registration

Note: This type of file does not contain any height information, so the default elevation is set to 0 meters.

; ENVI Image to Map GCP File
; projection info = {State Plane (NAD 83), 404, units=Feet}
; warp file: E:\monterey_sub2.img
; Map (x,y), Image (x,y)
5711285.2999      2114581.3280          1.000000          1.000000000000