Precision of GPS measurements and Map Coordinates.

When you hike with a GPS, the most interesting question is: how exactly can you find places in nature, when you have read their positions from a map in nature, and how exact can you measure a position to look it up in the map.

One can do a lot of theoretical considerations, or do measurements at home, but I found it interesting to compare results from a real tour, although I had only a few.

Details in the Map

One of the most important applications will be to take cabin positions from the map and store them in the GPS or to measure positions on a tour to find them up later. There are many sources of error, and one of the biggest are the markings in the map. The red triangle that indicates a wind shelter in the 1:100 000 Fjällkartan has a base length of little more than a millimeter. This is a bit more than 100 metres in nature, while the shelter itself has a size of maybe just 5 metres. And one even cannot trust on the correctness of the map in all cases.

Measurements by Mark and me in three places

Mark and me made different tours in April 1997, separated by two weeks. We both had measurements in three places, that is the cabins of Alesjaure, Abiskojaure and the wind shelter between Alesjaure and Abiskojaure. We compared the data, and in my opininion, they coincide very well. Someone with higher expectations, however, might be disappointed that the agreement is not better.

The difference "my measurement minus Mark's measurement" in metres in northern and eastern direction was:

Abiskojaure: E + 42 m, N + 4 m
Alesjaure:     E + 37 m, N + 97 m
Wind Shelter:  E - 6 m, N - 68 m

The picture below shows our measurements at the shelter beween Abiskojaure and Alesjaure.

Both measurements show an eastern deviation of 150 m from the position indicated in the map. I think that the shelter is not marked at the correct position.

The average deviation between our measurements at the three places is 42 metres. Part of it might be due to the fact that we did not fix in advance where to do the measurement.

Looking up positions in the map

Prior to the tour I determined positions of cabins from the map. I used the Fugawi Software, but as I mentioned before, this is not essential for accuracy, it just makes work easier.

After the tour I could compare 8 postion values. Two points had very bad agreement, namely Kebnkeaise Fjällstation, which I would explain by the size ot the facility and the fact that some of the satellites were screened by the building and mountains, and the Alesjaure shelter, as described above, which might not be marked at the correct place.

For the remaining 6 positions I calculated an average deviation of 49 m, corresponding to half a millimeter in the Fjällkartan. The distance from the object , when you have a difference of 50 m on every axis, is 70 m.

So, from this experience, I would answer the question "How accurate do you find a place in nature" with "70 m". It is still possible to miss a cabin if the visibility is only 30 m. But the compass is not more exact in any case, because already the map does not give the accuracy.

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