Closing Loops Across Separate Data Files.
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When you are closing a Project File that contains more than one survey file, the Loop Closer can close the files in two different ways:
1. Inside Files. When the Closer uses this technique, it only closes those loops found inside each file. This means that any loops that extend outside of an individual file are not closed.
2. Across Files. When the Closer uses this technique, it closes all loops both inside and outside of the files. This means that any loops, no matter how many files they may cross, will be closed.
Linking. The Loop Closer uses the way a project is Linked to determine if the files in the project should close across files. Links are used to connect together survey files that have duplicate station names and might interfere with each other. "Links" are a list of station names that are common between two or more files. When you use links, COMPASS assumes that the links should be the only connection between files and so it throws away all information except the links when it processes a new file. Normally, linking is used when you are connecting together surveys for several different caves. If is fairly common to have different caves with same station names. These duplicate survey names would cause the problems if you did not use linking.
Loop And Links. When you close a file with links, COMPASS assumes that the linked files could interfere with each other and should be kept isolated from each other. Therefore, it does not close any loops that cross between files. On the other hand, if there are no links, the Closer processes all the files together as though it were a single data file. As a result, all loops are closed together.
Compass treats fixed stations the same as links. In other words, if you have a fixed station associated with a file, Compass assumes that the file should be kept isolated from previous files and so it purges all station names except for the link.
Mixed Projects. COMPASS also allows you to combine survey files so that some of the files are closed in groups and some are not. This allows you to close some loops across survey files and some not. For example, you could have two caves, each consisting of several survey files. Obviously, you would want to close the loops inside the caves, but not want to do any closing between caves.
An Example. Here is a more detailed explanation of how Links effect what data is closed. The key is to understand that whenever COMPASS sees a Fixed Station or Link, it is a command to "forget" or "purge" all previous information except the Links or Fixed stations. Thus, by strategically placing your Links and Fixed Stations you can control what parts of the cave are closed together and what parts are not. Here is an example that illustrates the way it works:
As you can see, data files DataA, DataB, DataC and DataD are closed together. Also, DataE and DataF are closed together. If there are any loops between the two groups, they will not be closed together.
Normally, Links are only used to isolate two .DAT files or two groups of .DAT files. The only reason you do this is to keep from having duplicate station names. Usually, surveyors are careful not use duplicate station names in the same cave. So, the only time you have duplicate station names is when you are connecting two different caves together. Thus, the typical situation for using Links is connecting two different caves together. In my example, DataA, DataB, DataC and DataD could be different parts of the same cave and DataE and DataF different parts of a second cave.
Problems. There are a few situations where you could encounter a problem. For example, if you did have duplicate station names in the same cave, you wouldn't be able to close the whole cave at once. Also, there are situations where you might want to have a giant loop that includes several different caves connected together with an overland survey.
All these problems can be solved by making sure that you have no duplicate station names. One way to do this, is to prefix each station with a letter or number that would make all the station names unique. For example, if you had the stations A1, A2, and A3 in two different files, you could make them unique by prefixing them like this: 1A1, 1A2, 1A3 and 2A1, 2A2, 2A3. The COMPASS Editor has tools that can help you prefix stations in a survey.