v.transform

NAME

v.transform - Transforms a binary vector map layer from one coordinate system into another coordinate system.
(GRASS Vector Program)

SYNOPSIS

v.transform
v.transform help
v.transform [-y] input=name output=name [pointsfile=name]

DESCRIPTION

This program has been used to import vector files that were in scanner or digitizer (x,y) coordinates and to transform these into UTM coordinates.

OPTIONS

Flag:

-y: Suppress the printing of residuals or other information to standard output.

Parameters:

input=name: Name of the binary vector map layer to be transformed.
output=name: Name to be assigned to the resultant (transformed) binary vector map layer.
pointsfile=name: Name of a file containing transformation points, whose format is given below. Give a full path name for this file or it will be assumed to be located in the user's current directory.

The user can run this program non-interactively by specifying parameter values (and optionally, the flag setting) on the command line.

If the user runs v.transform without specifying program arguments on the command line, the program will prompt the user for inputs. When the program prompts the user for two sets of transformation points, the first set of points entered by the user should be in the coordinate system of the input map, and the second set of points should represent the corresponding geographic points in the coordinate system into which the map will transformed. A user must enter 4 to 10 of each set of points for the transformation to work correctly.

After the user has entered both sets of points, the program will show the amount of error associated with the transformation of the given points as the residual mean average (RMS). (An acceptable RMS for a 1:24,000 UTM map would be 1.2 to 2.4 (meters).) It will then ask if the transformation RMS value is acceptable. After an RMS is accepted by the user, v.transform will transform the binary vector (dig) map and its associated attribute (dig_att) file into the requested coordinate system.

Remember to run v.support on the output map.

NOTES

When rectifying a map to another coordinate system using v.transform, the user should specify the coordinates of between 4 to 10 points, and state these both in the coordinate systems of the input and output maps. The two sets of coordinates can be input to v.transform interactively, or from a file specified on the command line with the pointsfile option. The pointsfile option is especially useful when several maps in the same geographic area require transformation, as it eliminates the necessity for the user to repeatedly type in the same transformation coordinates.

A pointsfile file will contain between 4 to 10 lines; each line will contain the set of coordinate transformation points for the input map and the corresponding set of coordinates for the output map. The minimum number of lines for the transformation to take place is four.

The format of the pointsfile file is shown below:

 Input Map   Output Map

 x       y   x        y
 x       y   x        y
 x       y   x        y
 x       y   x        y

In the format shown above the x's and y's can be thought of as eastings and northings, depending on what coordinate systems you are transforming to and from.

An example of the pointsfile file:

 1       1       589000   4913000
 1       17000   589000   4930000
 17000   17000   610000   4930000
 17000   1       610000   4913000

Within the pointsfile file, numbers on a line must be spaced apart with tabs or blanks. The example shown above was used to convert a map in digitizer coordinates (range of 1-18000) to UTM coordinates within the UTM zone for the Spearfish sample data base location.

Because this pointsfile file is not your usual GRASS data file, the user will have to keep track of where it is on the system. When the pointsfile option is used on the command line to input the transformation points, the program does not ask whether or not everything is acceptable before converting the vector file and the attribute file.

The user is advised to run this program interactively with a specific set of transformation coordinates and to examine the resulting residuals, to determine how accurate the transformation will be (i.e., pick points with known values in both coordinate systems). After the residuals are acceptable, those transformation coordinates can be used with the program run non-interactively to transform other maps in the same geographic area.

WARNING

This is a general purpose program and can be fooled into giving low residuals. It is strongly suggested that any transformed map be checked for accuracy. The program assumes that the coordinate systems will be planimetric and has never been tested with negative values.

It is not recommended to use this program to transform maps from State Plane to UTM coordinates (or between any two different projections). Using v.proj will give better results.

AUTHOR

Michael Higgins, U.S. Army Construction Engineering Research Laboratory

Last changed: $Date: 2004/01/17 03:04:30 $