One Location, Different Coordinates

[StanB]

On my PC I have Delorme Street Atlas '05 with Earthmate GPS, downloaded 7.5 minute USGS topo maps with Global Mapper viewer, and a Garmin GPS. All three of these systems give me different Lat/Long coordinates for the same major highway intersection. The Street Atlas map coordinates indicate the location is about 700' south of the USGS map location, and the two GPS units indicate locations between the two and a little east. I double checked the appropriate datum in each case.

 

My question is; which of these should I believe? I understand that good Geocaching relies on GPS only, but as a newcomer I thought I could use a maping system to help me get close to a cache. Am I missing something here?

 

Note to Moderators: If I'm in over my head here please delete this post and tell me to just go find something. Stan

[+Renegade Knight]

One problem wiht maps is that the map itself is subject to limitations on it's accuracy. On Mapsource Topo when I'm in my house it shows me in my neighbors house.

 

You will notice when you are driving your GPS may not always show you as being on the road.

 

The base map has to be tweaked to match the coordinate system and it's normally close but not always. Just think of the map itself and the coordinate system as two layers lay on top of each other.

 

Different maps from different sources would have differnet people who meshed the map with the coordinates and you would expect different errors.

 

700' is a lot though and that's normally what you might expect with a datum error, or using the wrong coordinate type. The maps is expecting information in DDD.DDDDD and gets it's information in DDD MM.MMM for example.

[+NightPilot]

USGS topo maps are generally in NAD27. If you want to see your position on it, you probably need to have your GPS set to that. The maps are over 30 years old, so if the road has been moved for some reason, that won't be reflected on the map. And the map was never all that accurate, in many cases.

 

For caching, set your GPS to WGS84 to find the cache.

[+Poindexter]

Give us the three coordinates you have, the location of the intersection and the quad name.

[StanB]

My problem appears to be primarily with the downloaded topo map (from the UConn MAGIC library) and Global Mapper (DLGV32PRO). This map is the 7.5 minute Rockville, CT quadrangle with the datum NAD27. Longitude is very close but latitude coordinates from Mapper are off by about 0.12 seconds or roughly 710 feet to the north.

 

My reference is a road intersection in the NW corner of the map, just west of Rockville. It's the intersection of Windsor Ave. (Rts. 74 & 83) and Union Street, where Rt. 83 turns north. Both Delorme's Topo USA and Garmin's MetroGuide show similar coordinates but Global Mapper is off, and I really want to use the 7.5 minute topo map. Maybe I'm doing something wrong with Global Mapper, but I can't figure out what it is.

 

Topo USA: N41 52.104 / W72 27.874

MetroGuide: N41 52.108 / W72 27.882

Global Map: N41 51.980 / W72 27.940

 

I apologize for the long post. Any help will be appreciated.

Thanks,

Stan

[+Renegade Knight]

Looks like you have the maps set to different datums.

 

If you take a WGS 84 coordinte and plut it into a NAD 27 map the map assumes you are putting in a NAD 27 coordinate and so it will be off. If you translate the datum first then the maps will show the cooridnates in the same spot.

[JohnX]

Hey Stan,

 

I am not a moderator and you question was a good one. Don't worry about it.

 

There was a recent thread that gave a lot of good advice about finding your first cache. The best was to go to a park. Mark a waypoint in your GPS, walk away and try to find the same place using only the GPS. On my first caching trip my GPS unit had me going in circles because I didn't realize it has to be MOVING to show direction. I would stop, let the GPS "settle" and go off in the direction the arrow was pointing. The wrong direction. And then I would do it again and..... I went home empty handed on that first trip.

 

So what I am saying is ignore the maps for now and go out and find something. And remember to have fun.

[+Poindexter]

The problem is with the georeferencing information within the tif file. Also, the associated tfw file is incorrect as well. When I import the tif file into Oziexplorer (not using the tfw file) it auto-calibrates it using WGS84 datum. This gives incorrect coordinates because the map datum is NAD27. In Ozi, I use the "check calibration" feature and change the datum to NAD27 and "save" and then all is OK. If I import and auto-calibrate using the tfw file I can see that the calibration points are in the wrong place. The longitude is correct, but latitude is way off. First time Iv'e run into this problem.

[StanB]

Thanks for checking, I'm relieved to know it wasn't just my own screw-up. Maybe I'll hack into the tif file to see if I can change it. My new eTrex should arrive in a couple of days and then I'll get out of the house. Stan

[+Poindexter]

I tried that. The georeferencing data is not in plain text. There is a reference to WGS84 at the very bottom of the file and I changed it to NAD27 but it had no effect when importing to Oziexplorer, I still had to manually change the datum. The tif file is read-only so you'll have to remove that attribute before making any changes to the file. I also downloaded the files from GISDataDepot as I have an account there. The tif file from there has no georeferencing data in it so you must use the world file (tfw) that comes with it and it is screwed up as well. Weird

[3D-cachin]

Here is a good description of where the 700+ ft error comes from: utm-faq

[+Poindexter]

Here is a good description of where the 700+ ft error comes from: utm-faq

We understand that it is a datum problem but in this case it's not an end user error. We are using programs that autocalibrate these maps using the supplied geo-referencing data however that data was incorrect in this case so we must correct it manually. For those that are using these programs in the future with these geotiff files I would suggest that after the autocal is done that you immediately check for error by placing your cursor over the utm tic marks on the maps and make sure the coordinates are correct.

[+Greymane]

I have found this in Memory-Map as well. It is a general assumption that most maps are in WGS84 format now. Some websites, however, still have older NAD27 calibrated maps mixed in with newer NAD83 (WGS84) maps. You have to be careful of the source. If you need to calibrate the map yourself (or adjust the calibration), most newer maps have markings for both the NAD27 datum and the NAD83 datum.

[+NuvoSeeker]

It is a general assumption that most maps are in WGS84 format now.  Some websites, however, still have older NAD27 calibrated maps mixed in with newer NAD83 (WGS84) maps.  You have to be careful of the source.  If you need to calibrate the map yourself (or adjust the calibration), most newer maps have markings for both the NAD27 datum and the NAD83 datum.

I understand that NAD27 is old-and-different, but am confused about the newer formats... are NAD83 and WGS84 the same thing ...?

[+Markwell]

North American Datum 1983

World Geodetic System 1984

 

In North America they are virtually the same. From what I hear in other parts of the world that's not quite as true.

[+NuvoSeeker]

North American Datum 1983

World Geodetic System 1984

 

In North America they are virtually the same.  From what I hear in other parts of the world that's not quite as true.

Got it ... Thanks!