Latitude/Longitude and UTM conversion in compass

[alanf]

I am trying to overlay a compass plot on Google Earth.

I have a long/lat coordinate from my GPS 87, 16, 36.5 / 20, 29, 1.25

That plots fine and is pretty much where I expect it to be when I look at Google Earth.

In compass I fix a station to this coordinate, but need to select "Use UTM" and do a conversion with the geocalculator. This gives me, E 471,133.3 , N 2,264,897.0 Zone 16

I generate the KML and open in Google Earth, its close by but its not right. The fixed station is about 20 - 30m meters Northish from my GPS coordinate.


I have spent hours fiddling with this. Any ideas?

[Webmaster]

Make sure you're using the same datum for all of them, NAD27 or NAD83 for instance. Using different ones will cause it to be off.

That is if the coordinates from your GPS are recorded in NAD83, you need to make sure that Compass expects coordinates in NAD83.

I don't know what datum you're using, but the NOAA site for instance shows a 60m shift for those coordinates between NAD27 and NAD83. http://www.ngs.noaa.gov/cgi-bin/nadcon.prl

[jlillest]

GPS devices store points internally in UTM format, try getting the UTM coordinates directly from the GPS device.

My first thoughts about the 20-30m difference is that could easily be a datum error. Assuming that this is a point in Mexico (I'm not on my laptop where all my GIS software is), your topo maps are probably from INEGI and they are drawn in NAD83 (close enough to WGS84 to prevent most campfire arguments). Check to make sure that Compass isn't giving you a NAD27 coordinate, which is Compass's normal format because US topos are in the NAD27 datum. Select WGS84 and see if that solves it.

If that's not the solution, it may just be rounding error or a problem with the conversion process to Compass. I pulled my hair out a few years ago when all the caves in my project area were displayed on the opposite side of the road. I always figured that I made an error somewhere, but having still not solved it after all these years, I'll feel better if it is in fact a problem with Compass. I won't hold my breath though, it's a good piece of software.

-Jon

[alanf]

Thanks guys, NAD83 works a treat I will play with WGS84 as well and look closer. Quite amazing what information and tools we have at our fingertips these days.

[reato]

Hey, Alan,
WGS84 is what you have to use.
I agree with jlillest, if you can is better to obtain utm coordinates directly from your gps.
You also have to check what is the error of waypoint in that moment.
U.S. own satellites, sometimes they give you a small error, some other time a bigger one. Check on your device, if the error is too big (100 ft) you can decide to take again the same coordinates few days later.

[adam0321]

just use mgrs( military grid reference system) if we can figure it out its dummy proof. converts very easily to

[adam0321]

sorry a mgrs should look somethng like
38s LT1234
5678

the 38s is the map sheet name the LT is the grid zone designation and then you can have from a 4 to 10 digit grid. You use a military protractor and ad the GM angle and that will give you either a magnetic or grid heading depending on how you are converting it. All GPS systems should have MGRS

[jlillest]

sorry a mgrs should look somethng like
38s LT1234
5678

the 38s is the map sheet name the LT is the grid zone designation and then you can have from a 4 to 10 digit grid. You use a military protractor and ad the GM angle and that will give you either a magnetic or grid heading depending on how you are converting it. All GPS systems should have MGRS

How does this differ from UTM? The way I understood it, UTM was the system the military used to make things easier for nav.

The unfortunate part is that (unless it's equivalent to UTM) Compass doesn't do MGRS. USGS topo maps are NAD27, GPSes do WGS84/NAD83 and except for those rare times that you're working with state plane data, you only need to convert between the two. For most everything in the US & Mexico, we only need the two datums plus the occasional conversion to Lat/Lon.

[adam0321]

utm and mrgs are very simmilar but not the same. and a compass to my knowledge only gives you an azmuth in dagree or mil if you have a military compass. you can plug two points on a mgrs map then draw a line between them connecting the two dots, place the protractor on the map and read the dagree off the protractor. next you look at the declination diagram on the map to add you gm angle. this is your heading. the protractor and declination charts are used to convert grid north(as read on the map) to magnetic north(as read on your compass). or you can plug the mgrs into your gps and it will take you there.

[adam0321]

this will clear it up a bit. the first is a demonstration of protractor use the second is mgrs on a map. if you are familiar with utm it will look very familiar

http://www.youtube.com/watch?v=rD0d2...next=1&index=6

http://www.youtube.com/watch?v=sbzwI...eature=related