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General Category => General Discussion => Topic started by: magellan on June 06, 2009, 08:17:16 PM

Title: Topo maps Rock
Post by: magellan on June 06, 2009, 08:17:16 PM
I just finished my Spring Summit trip (a five week long outdoor trip for twelve credits at Brigham Young University Idaho), and I have to say that I love the maps. We were on a survival course with Dave Holiday and couldn't find where the water stash was (Dave wasn't the one who hid it or he would have found it easy), but the topo maps helped show our location, since we were already off our paper map, and we were able to get on the right track after going 12 miles off course.

I have to say though that I found the state specific maps more accurate than the northwest/southwest topos. Those were as much as 20-40ft off the actual elevation whereas the Utah topo only varied 1-2ft. I don't know if the information was gathered from different places, but I sure liked the Utah topo. Good job guys.
Title: Re: Topo maps Rock
Post by: maps4gps on June 07, 2009, 04:19:24 PM
Good thing it was not a Summer survival course in 110 degree weather or we would be hearing about it on the news  - no paper maps, 12 miles off course.   Whoever was leading that sure needs to get some basic survival training.

The creator of the regional topos (NW, SW etc) mentioned he constructed contour lines from an elevation grid with 30 meter spacing; eveyone else is using data with a 10 meter grid spacing.

Given all the factors involved and their ranges of accuracy, you were extremely fortunate to be 1-2 feet off.   Was that in the horizontal direction or the vertical?
The rule of thumb for the vertical accuracy on the GPSr is 1.5-2 (some even say 3) times the horizontal accuracy; so with a circle of probably error of 10 - 20 feet, the vertical will be 15, 20, 40 or maybe even 60 feet.  Most of the time you will be closer (even much closer), just like finding a cache when the GPSr says you are one foot from it
Only real way to judge the accuracy of the contour lines is to compare with surveyed benchmarks (being careful to use only the high quality ones as many are off by quite a lot) and maybe the contours on the published USGS topo maps as most of the elevation grid data are produced from them.