Fixed Stations And Real-World Coordinates.
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Fixed Stations. It is sometimes useful to set one or more stations in a cave to a fixed location. Compass allows you to tie any station in your surveys to fixed location that consists of an East, North and Vertical offset. In Compass, fixed stations are applied at the Project level rather than the Survey level. This allows you to have different projects with different fixed coordinates while using the same survey data. For example, you could have version of a project where one is tied to GPS coordinates, while the other is tied to a land survey.
How Fixed Stations Work. Tying a station to a fixed location has the effect of making all the stations in the cave relative to that location. For example, if you tie the entrance of a cave to a GPS location, all the stations in the cave will have GPS locations. Thus, with a GPS receiver, you can locate the surface positions of rooms and passages in the cave. You will also be able to tell the elevation of various rooms and passage above (or below) sea level.
Multi-Cave Systems. If you set the entrances of more than one cave to GPS locations, you will be able to view a combined plot of all the caves without having to make overland surveys between the caves. This makes it much easier to combine multiple caves into a cave system.
Multiple Fixed Stations. If you set more than one station in the same cave to a fixed location, COMPASS will use its loop closer to compensate for any error that occurs between the GPS locations and the survey. This can make the plots more accurate. Warning. GPS receivers can have relatively large errors at certain times. For example, during the time of an international crisis the military may intentionally degrade the accuracy of the GPS system. Be sure that you understand the workings of your GPS receiver before setting multiple stations. If you set two or more inaccurate fixed stations, it can cause distortion of the plot.
Other Coordinates Systems. Fixed coordinates don’t have to be derived from GPS. They can be any other coordinate system you choose. For example, you could take your coordinates from benchmarks, topo maps, land surveys, precision (theodolite) surface surveys or even use your own coordinate system derived by taking measurements from a local landmark.
Fixed stations are usually, applied to entrances, but they are the only stations that are accessible to the surface. However, if you have "cave radio" coordinates or fixed coordinates, you can apply them to station in the interior of the cave.
Here is more detailed information about fixed stations: