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GPS, maps, coordinates and datums oh my!

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Offline badknees

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GPS, maps, coordinates and datums oh my!
« on: April 14, 2018, 09:52:16 AM »
Got coordinates for the Jenga cairn? I'd like to guess how much we came up short...

I recorded a waypoint on my GPS:

UTM 13R 0701779 E 3234584 N NAD27
LAT 29.225555 LON -102.92410266521527 NAD27

The Lat-Lon is calculated using an online utility. The waypoint was recorded using UTM.

I was looking at Jeff B's post and the coordinates for the "Jenga Cairn", and it brought up a point that may be frivolous to most, but here you go.......

If you use these coordinates in a GPS or mapping program that is configured for WGS 84 (tends to be the default), without changing the datum first, the waypoint will show up about 100 ft from its location. In this case to the ESE.

While this may not be of consequence to most of us, if you are trying to locate something within +,_ 10 meters, it is imperative that the datums match.

NAD27 is a bit outdated and has more error than  WGS 1984 and NAD 1983 coordinate systems.

It's probably best to use WGS 1984 or NAD 1983, as they are earth centric in origin. NAD 27 is based of a survey point taken in Kansas and  the Clarke 1866 spheroid representation of the earth's shape.

Unfortunately a lot of the older topo data is NAD 27 data. - North American Datum of 1927.  It is the datum used for most USGS topographic maps printed before 2009. NAD83 is used on maps printed after 2009,  If you are using GPS along with a topographic map, you should make sure that the GPS datum matches the datum used on the map. The default datum for most GPS units is the World Geodetic System of 1984 (WGS84), which is virtually identical to NAD83.

So what does all this mean to you?

Nothing if you don't use GPS or care about precise location of a geographical feature!!

If you do use GPS, it could mean errors up to 100+ feet if input coordinates are a different datum than the map, or the GPS device mapping those coordinates. (see the attachment as an example)


Recommendations:

1. Ignore everything I said above and keep doing what you've been doing. For the most part, those that use maps, GPS and their powers of observation while hiking won't be affected by this difference. If you were a Geo Cacher, and 10-20 ft made a difference in finding something, it would be best to use matching datums.

Or....... if you are interested in this kind of thing, be aware of the errors that can be observed by mixing datums.

This post was probably a waste of your time, but I'm trying to get to 4000!



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Offline jeffblaylock

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Re: GPS, maps, coordinates and datums oh my!
« Reply #1 on: April 14, 2018, 10:22:18 AM »
Unfortunately a lot of the older topo data is NAD 27 data. - North American Datum of 1927.  It is the datum used for most USGS topographic maps printed before 2009. NAD83 is used on maps printed after 2009,  If you are using GPS along with a topographic map, you should make sure that the GPS datum matches the datum used on the map. The default datum for most GPS units is the World Geodetic System of 1984 (WGS84), which is virtually identical to NAD83.

And that's exactly why my GPS was set to NAD27 for this trip. I had my paper USGS quads with me, and they are all based off that outdated projection.

I try to remember to include the datum (NAD27, NAD83, WGS84, etc.) whenever I provide coordinates because just the simple UTM or lat-lon numbers by themselves can produce various results depending on the GPS/map datum.

For the most part, hiking and backpacking don't require a lot of precision in waypoints. In this particular case, you absolutely will not miss the cairn if you're hiking on the Marufo Vega Trail. It is literally in the middle of the trail, and it is the tallest and biggest object for hundreds of feet in any direction. But, that hundred or so feet difference between datums could lead one up an incorrect wash on the way to Slickrock Canyon, and thus not to Slickrock Canyon.

So, badknees is right. Best practice is to make sure your coordinate datum and GPS/map datum line up. Also provide the datum with any coordinates you publish here or elsewhere.

Now, we just need to restart the whole UTM vs. Lat-Lon fight.
Jeff Blaylock
Austin, Texas

"We'll be back, someday soon. We will return, someday, and when we do the gritty
splendor and the complicated grandeur of Big Bend will still be here. Waiting for us."--Ed Abbey

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Offline badknees

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Re: GPS, maps, coordinates and datums oh my!
« Reply #2 on: April 14, 2018, 10:33:42 AM »


Now, we just need to restart the whole UTM vs. Lat-Lon fight.

Oh no, not me!
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Online mule ears

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Re: GPS, maps, coordinates and datums oh my!
« Reply #3 on: April 14, 2018, 02:14:31 PM »
Great info and possibly why on my last trip I was scratching my head when the coordinates for some springs did not line up with my location and maps, I probably had my Caltopo maps set for WGS1984 instead of NAD27.   :eusa_doh:
temperatures exceed 100 degrees F
minimum 1 gallon water per person/day
no shade, no water
http://40yearsofwalking.wordpress.com/

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Offline badknees

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Re: GPS, maps, coordinates and datums oh my!
« Reply #4 on: April 14, 2018, 02:19:38 PM »
Great info and possibly why on my last trip I was scratching my head when the coordinates for some springs did not line up with my location and maps, I probably had my Caltopo maps set for WGS1984 instead of NAD27.   :eusa_doh:

All of the spring data from the "database" was projected to UTM NAD83. That's almost identical to WGS 84. So....maybe it was the opposite.. .CalTopo for NAD27?
In addition, the quality of some of the waypoints may be in question too? The survey was done in '06 and no telling what was used to record the locations.
Not all those who wander are lost.
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Offline House Made of Dawn

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Re: GPS, maps, coordinates and datums oh my!
« Reply #5 on: April 14, 2018, 04:04:37 PM »
This post was probably a waste of your time...

Nope. I learned something!
Good for me; good for you.

...but I'm trying to get to 4000!

 :icon_lol:
"The trick, William Potter, is not minding that it hurts."

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Offline House Made of Dawn

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Re: GPS, maps, coordinates and datums oh my!
« Reply #6 on: April 14, 2018, 04:05:39 PM »
Great info and possibly why on my last trip I was scratching my head when the coordinates for some springs did not line up with my location and maps, I probably had my Caltopo maps set for WGS1984 instead of NAD27.   :eusa_doh:

That's exactly what I was thinking, ME. Probably explains the minor discrepancies we had.
"The trick, William Potter, is not minding that it hurts."

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Offline fartymarty

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Re: GPS, maps, coordinates and datums oh my!
« Reply #7 on: April 15, 2018, 12:55:03 PM »
This is all great information, and very interesting.

However (sorry, just a bit off topic, apologies), I wanted to highlight these statements:

Recommendations:

1. Ignore everything I said above and keep doing what you've been doing.....
This post was probably a waste of your time, ....

I just wanted to note how very rare and unusual such comments are on today's internet. Usually posters and bloggers
seem to think their musings and information should be taken as though it came from on high benevolent mystic ruler gods.
They frequently say stuff like "it's a no brainer" as though using your own brain is a bad thing and letting their superior intelligence
do it all for you is what you should do.
 
This on the other hand is very interesting and is something I'm going to play with on those rainy days when I sit around on the computer
all day with my GPSs at hand, looking at maps and Google Earth, doing interesting stuff. (those days where my wife says to me around 4:45 PM, "Are you just not going to get dressed at all today or what?")
..and I apologize again for the diversion from the topic at hand, but the rarity of those remarks just caught my eye and I couldn't let them go.  :)

Please continue, so that I may learn. :eusa_think:
Fort Worth

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Offline badknees

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Re: GPS, maps, coordinates and datums oh my!
« Reply #8 on: April 15, 2018, 05:02:55 PM »
For anyone that is interested in converting between datums and from Lat/Lon to UTM and vice versa.

I use this tool. It does single points or does batch files from .csv files

http://seisware.com/products/coordinate-converter/

You don't have to register for anything...
« Last Edit: April 15, 2018, 07:43:42 PM by badknees »
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Through the Mirror
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Offline Casa Grande

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Re: GPS, maps, coordinates and datums oh my!
« Reply #9 on: April 15, 2018, 09:13:58 PM »
My GPS unit crapped out after 15 years.   Gonna have to replace.  Any suggestions on one that is relatively inexpensive,  yet effective?

sent from my Note 8 using the Big Bend Chat mobile app


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Offline badknees

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Re: GPS, maps, coordinates and datums oh my!
« Reply #10 on: April 15, 2018, 09:33:15 PM »
My GPS unit crapped out after 15 years.   Gonna have to replace.  Any suggestions on one that is relatively inexpensive,  yet effective?

sent from my Note 8 using the Big Bend Chat mobile app

Garmin 64s is a pretty solid unit.  Lacks some of the High $$$$ feature,s but for $225 ish you get a good receiver, electronic compass and good storage.

All depends on what you need. The big screen loaded models get pretty pricey.
Not all those who wander are lost.
J.R.R. Tolkien

Through the Mirror
http://mirrormagic.com

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Offline presidio

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Re: GPS, maps, coordinates and datums oh my!
« Reply #11 on: April 15, 2018, 09:37:25 PM »
My GPS unit crapped out after 15 years.   Gonna have to replace.  Any suggestions on one that is relatively inexpensive,  yet effective?

Anything from Garmin, as long as it has a data port.

I avoid to the extent possible all the crap that gets build into these thing now (even a camera?). So no electronic compass, barometer, altimeter, thermometer or anything else that doesn't directly relate to coordinates. What I can't avoid I disable.
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Re: GPS, maps, coordinates and datums oh my!
« Reply #12 on: April 15, 2018, 11:32:11 PM »
I got a Garmin 64s last year for what will be my last dedicated GPS unit. Replaced an ancient eTrex. it's OK, but the UI is still somewhat counterintuitive to me. It's a redundant device for me, as I'm usually already carrying a phone and Spot tracker device. (And for anything more than a half day hike, or longer but on a well defined trail,  a map + compass).
Serious question because I do not know: is "datums" a correct plural? Like "stadiums"?

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Offline badknees

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Re: GPS, maps, coordinates and datums oh my!
« Reply #13 on: April 16, 2018, 12:01:20 AM »
I got a Garmin 64s last year for what will be my last dedicated GPS unit. Replaced an ancient eTrex. it's OK, but the UI is still somewhat counterintuitive to me. It's a redundant device for me, as I'm usually already carrying a phone and Spot tracker device. (And for anything more than a half day hike, or longer but on a well defined trail,  a map + compass).
Serious question because I do not know: is "datums" a correct plural? Like "stadiums"?

Plural of datum is actually data, not datums
Not all those who wander are lost.
J.R.R. Tolkien

Through the Mirror
http://mirrormagic.com

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Offline catz

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Re: GPS, maps, coordinates and datums oh my!
« Reply #14 on: April 16, 2018, 09:26:27 AM »

And the proper, but rarely used, plural of stadium is stadia.
Wake me when it's time to go.

 


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