Understanding formats for text files containing GCPs

You can import or export ground control points (GCP) to a text file. Each line in the text file contains the data for one GCP. The line is divided into fields separated by spaces, tabs, or commas. Each field contains a piece of information about the GCP.

When you import or export the GCPs to a text file, you must identify what the fields contain and the order in which they appear. You do so by representing the order and the contents of the fields as a string, or format.

The format characters are shown and described in the following table.
Table 1. Format characters
Format character Description
ID Identification number of the GCP.
P Pixel location of the GCP on the uncorrected image file.
L Line location of the GCP on the uncorrected image file.
X Geocoded x-coordinate.
Y Geocoded y-coordinate.
Z Geocoded z-coordinate.
eP Estimated error of the pixel coordinate measured in pixels.
eL Estimated error of the line coordinate measured in pixels.
eX Estimated error of the x-coordinate measured in pixels.
eY Estimated error of the y-coordinate measured in pixels.
eZ Estimated error of the z-coordinate measured in pixels.
S Status of the point. Valid values are as follows:
  • G: Ground control point (GCP)
  • I: Inactive point
  • C: Check point

For example, the string IDXYZ represents the layout where ID is the GCP ID, X, Y, and Y are the GCP's geocoded x, y, and z coordinates.

The line in the text file and the format must match. Lines that do not match the format are ignored.

Fields can contain text, alphanumeric values, integers, decimals, or exponential values (for example: 1.234+E05). Latitude and longitude values must appear in decimal degrees (123.5) rather than DMS form (123 30 00). To include a phrase in a field, enclose it in double quotation marks.

If the field contains a numeric value ending with text, the text is ignored. For example, N or W in latitude and longitude values are ignored; instead, use a minus sign.

Tip: Because latitude (north/south) usually appears before the longitude (east/west), use YX instead of the more common XY.

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