CLOSE LOOP Program Overview
When doing field work for mapping, a standard surveying technique is to work in loops (i.e. to return to the starting location). This is a powerful surveying technique, but when doing this cumulative errors usually result in your field map's ending location and starting location, which should coincide with the each other, ending up quite far apart. The purpose of the CLOSE LOOP program is to correct this problem in a consistent manner.
To use the program, one digitizes into OCAD the loop as it appears on the field work's template ; except instead of ending where the field work ends, one more point is added, ending at the starting location. This is exported in DXF format, and the program is run. The extra "leg" is used as an "error vector". This "error vector" is then proportioned out at each of the turning points on the line, resulting in a corrected loop closing on itself. A new DXF file is created, which can then be imported back into OCAD.
Note that the loop does not have to be one feature like a path, but can consist of a collection of features, fences, roads, streams, etc. and even point features. One just connects these features together with a single line feature, then places the proper features on top of the corrected loop and when finished removes the corrected loop.
Also, to use the program one does not have to return to a starting point, any two known locations will do. (Although, OCAD 6's method of adjusting the template works well for distant starting and ending locations.)
Detailed Instructions on the Program's Use
The following assumes the user is working with OCAD 6.
The first thing required is to open your map and template in OCAD, and align the template. Now digitize in your "loop" with a line symbol which is not being used for anything else. Use the Straight Line Mode. This program will not work with other modes.
At this point export this single symbol naming the output file Line.ocd. Open Line.ocd and export in DXF format naming the output file Line.dxf using the default settings. This should be the default name, if you created the OCAD file with the name Line.ocd. Before exporting make sure that there is only this one symbol in the entire OCAD map. The program only works with the first line it finds in the DXF file.
Place the Closelp.exe program in the same directory to which you saved Line.dxf. From the Window's start menu choose the Run option. Either typing in the path to Closelp.exe or use Browse to point to the Closelp.exe program and click OK. The program will look for the file, Line.dxf in the same directory with itself, load it, make the corrections, and write the corrected loop back to disk with the name, Newline.dxf in the same directory.
At this point you import Newline.dxf back into Line.ocd map. Put in zeroes for the Horizontal and Vertical Offsets. The new closed loop will come back in with a gray color, which can be difficult to see. It might help to hide the template to make it easier to spot. If everything worked, the last "leg" of the loop will be dropped and corrections will be applied uniformly to the other turning points.
The above step is really not necessary, but makes for a good quality control check. You can proceed directly to import Newline.dxf into the OCAD map you want it to end up in. You can then use the solid Edit Point tool to drag it to where you want it. The imported line will come in with an undefined symbol. Use the Change Symbol tool to make it into the symbol type that you want.
Page updated: 12/18/200498