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 ASCII (dig_ascii) vector map and its associated attribute (dig_att) file into the requested coordinate system. 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. 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. If this program is being used to transform maps from State Plane to UTM coordinates, and vice versa, users should be aware of the following points. This program will work better with State Plane zones that use the Transverse Mercator projection. Those are states that have their state zones splitting the state vertically, like Illinois. This program will not work as well with states that use the Lambert Conformal Conic projection. Those are states that have their state zones splitting the state horizontally, like Wisconsin. It is also best to keep the area being transformed relatively small. Source.
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