Which Location? Ground or Map Position?
As Geographic Information Systems become more common, and Global Positioning Systems become cheaper (we wonder if they will soon be on bicycles) a few issues are developing.
One of the interesting ones is the difference between the position on the ground and the position on a map. They are simply not the same in principle, and do not need to be (although we all wish they were the same). This situation will persist even after they remove the "fiddle" from the GPS systems or automatic real-time correction becomes common.
The map is a projection with the distortions that all projections involve, but there will always be some sort of inaccuracy in the map. More to the point, maps in the next few years are about presentation and sharing information. As organizations cooperate and share information consistency is often far more important than accuracy. Even if you knew the correction which should be applied to a map error it may not be possible to make that change. Too many other people will be involved.
One simple and practical solution may be to keep both locations as separate items in the database, the ground position which allows you to relocate that plot, and the map position which allows you to share or match information on a map base. If you are very lucky, of course, they will be very close to each other.
An example might be useful. You know that you are just inside a riparian border (it is marked well) and know the GPS position that goes with a set of sample data. When you plot this position on the map, it falls outside of the riparian border. What can you do?
If you are not allowed to change the map would you want that data associated with the area outside the riparian zone? Why not locate the position on the map that you believe best represents the area where you measured data? That way, if other information is retrieved from the GIS system it is associated correctly with the riparian zone. With luck, you will only be working with one map base. Otherwise, the problem really multiplies.
As we become more connected to other sets of information, particularly GIS information, the opportunity to "correct" things decreases, and at least takes a lot longer. Compatibility and relative consistency of information are often more important than "more correct" information.
As we link more sets of data together it gets constantly harder to correct bad procedures. In the next few years, a lot of bad decisions are going to be frozen into place.
Now is a good time to make sure that people who actually know what they are doing are involved with plans, definitions, and procedures. Errors will be harder to correct in the next few years as GIS systems tie more of us together more tightly. The window of opportunity will close quickly.
Originally published October 1999
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