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Map Datum


chrisski

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Map Datum


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Here’s something I’ve been dealing with while out prospecting is Map Datum. For those who download maps off the USGS site, you’ve probably noticed that the newer maps like the 2014 have some imagery built into them, but are lacking on other things such as the new maps don’t show mine location, and they don’t the type of roads you’re on. The newer maps make a jeep trail look the same as a two lane paved road, which is not good for planning your trip to the remote location, the older maps still have accurate road type and mine shafts and prospects on them.



With these older maps, they are often a different datum, or coordinate system. So, on a lot of maps, they tell you how to translate from the old coordinate system to the new coordinate system. For example, on the map I’m using it says to convert WGS 84 MGRS to NAD 27 MGRS, go 17 meters to the west, and 213 meters to the south. So, if you had the wrong datum in the GPS, you could be that far off. I’ll bet many, many people on this forum actually used the map that this example came from.



To show you how far off that is, If I’m trying to stake a 20 acre claim off an old grid I found off of research on the short side of my 20 acre claim, and I’ve put WGS84 datum in my GPS, I will actually by 213 meters off, or possible an entire 20 acre rectangle off. The short side of a 20 acre claim is 193 meters. In AZ, the distance the datum is off is not as severe, It can be around 20 meters, but when that creek bed is on the border even the wrong datum in the GPS will put you mining the creek that is actually in the adjacent claim.



If I get a grid location on my protractor for a mine off the 1981 or 1969 map, I need to put it in the correct datum, or I may never find it. If you did enter it wrong, hopefully your map reading skills are well enough that when you get to the location you’ll notice where you are on the map is not what the GPS is saying and adjust.



I deal with maps a lot and what I’ve noticed is the places I want to go are never on the same map. For example, I printed up the USGS 1:24k maps for Lynx creek, but that is actually on the corner of four maps. In those maps, it was the same datum. For some other maps, particularly any special maps, the datum won’t match. All maps are moving towards WGS 84, but not all of them have been revised and reprinted. When that’s the case I have to either convert the coordinates to one datum, or keep swapping the GPS back and forth.


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I've attached a couple of pics that show why I like to use the older maps. The USGS for one reason or another does not put the detail into the newer maps they used to. This is a map of a random place I chose off of one of the maps I downloaded. The new map does not show the mines, tailings, prospects, and shafts like the old map does. Very rarely do I swap to the newer maps. I was looking at an area that had a reservoir built, and sometimes there's been urban development, but not often.post-26611-0-70007900-1423100641_thumb.jpost-26611-0-97474500-1423100642_thumb.j

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Yup ... You do need to know which datum you are using ... I found the same problem in Northern Maine last year ... I had a map I have had for 20 years or more and wanted the adjacent map ... not only did they not line up but the trails (log roads mostly) noted on the old map were no where to be seen on the new ones. I did finally find another hunter with the old version of the map I was needing ... and he had two of them ... I am now the owner of one of them ... a little tattered but very usable for my purposes.

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My older USGS maps show roads trails and mines the new maps don't show at all.

Could it be that they do not want us to know about them so we only use the main routes they want us to use!

Bob

BIngo, I think that is it, keep the humans out only animals allowed!!!

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Part of the plan to scare us all into living in the city with all the paved roads ... then the guberment can conduct all their clandestine activities in private without the prying eyes of patriots! Hang on to those old maps ... they may not be gold but they sure could be considered so under the right conditions! Just sayin'! :old:

Mike F

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I wanted to post these two pictures of the map I used in the first post. I'm sure many here have spent time there. One shows the explanation on the datum conversion. The other shows where the map is from.

post-26611-0-45388700-1423184384_thumb.j

post-26611-0-26147200-1423184385_thumb.j

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I deal with maps a lot and what I’ve noticed is the places I want to go are never on the same map.

It's been that way for at least the last 25 or more years that I know of. With all the various maps I own, I discovered that different cartography companies have their own ways in how they print and mark their maps. As an avid dirt bike rider here in AZ, one of the first things I did (pre internet, pre GPS) before getting serious about riding the AZ back country, was to get a hold of as many old maps that I could, so I could plot my exploration routes. Before we had GPS's, we used to use "roll sheets" that we mounted to our handlebars that we marked with detailed directions, landmarks, turns, mileage, etc. etc. to help us navigate the back country. When the first handheld GPS's came out, I bought one and thought I was in heaven. No more roll sheets! Luckily the original Garmin GPS's came with a map that matched trail for trail that many of my old NF and BLM paper maps showed. To this day I still carry (as a backup) one of my original Garmin GPSMAPs that I purchased almost 20 years ago... just because it has all the old pack trails, landmarks, water tanks, springs, mines, mountain tops, etc. that none of the newer GPS (and paper) maps contain.

For many years, I've often said that there is some sort of government conspiracy to "keep the indians on the reservation". In other words, the government don't want you or I out exploring on our own public lands! So what do they do? They dumb down the maps to only show a few trails here and there that "they" think you or I should only be riding. A few years back, I spoke with one of the Delorme cartographers that was in charge of mapping the AZ back country. I asked him why is the government instructing you guys to "dumb down" your maps. He simply stated... "they don't have the manpower or resources to properly "oversee" your activities. They figure if they can keep you on the major trails, they can "manage" you better." Are you fricking kidding me? :grr01:

Do the stupid SOB's think that by not including any of the old mining pack trails, mines, or landmarks on their current maps, that somehow I'm not going to be able to find, ride, or explore them? I guess they're not familiar with Google Earth, or any of the previous decades of earlier maps that were produced showing those trails and landmarks. I think it's funny! No wonder I rarely ever see anybody out in the boonies where I prospect. They don't have a clue of how to get there. :rolleyes:

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