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Old 10-21-2014, 09:31 PM   #1
aws
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Default GPS Users - Do You Know Your DATUM?

I have recently been involved in a discussion on a different forum regarding GPS coordinates, units and Datum. It began as a "which do you prefer... lat/lon or utm" and it appears that there are a lot of people that don't understand datum. If you try to pinpoint your location on a map using GPS coordinates (either lat/lon or utm - doesn't matter) and your GPS is using a different datum than your map, you could be miles off.

So, my question is how many people understand datum and why it is important in emergency situations?
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Old 10-21-2014, 09:41 PM   #2
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Default Re: GPS Users - Do You Know Your DATUM?

Please enlighten us.
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Old 10-21-2014, 10:05 PM   #3
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Default Re: GPS Users - Do You Know Your DATUM?

Lost me.
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Old 10-21-2014, 11:23 PM   #4
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Default Re: GPS Users - Do You Know Your DATUM?

"Simply" put, "A geodetic datum is the tool used to define the shape and size of the earth, as well as the reference point for the various coordinate systems used in mapping the earth". There are hundreds of different datums. Some are vertical datums and others are horizontal datums. We will focus on horizontal datums. Because different datums are based on different reference points, one specific location will have many different coordinates based on the different datum used.

If you are not confused yet, just keep reading.

Let say you have a shiny new GPS unit and you use it to get your current position coordinates. Most likely your new GPS is set to the latest WGS84 datum as default (not guaranteed though). You also have a brand new USGS Quad map (that probably hasn't been updated since the 1940's or 1950's). The WGS84 datum was not in use then (not even established until 1984) - the datum used for the map was probably NAD27. If you try to plot the coordinates you got from your GPS onto your map, you will be off. Some datums are only off by several meters, others can be much much more depending on where you are in the world and which datums are being used.

So how does this really affect me, you ask?

Lets say you are riding solo and crash your shiny new 2015 KLR650 somewhere out in the "middle of nowhere". You need help. You pull out your cell phone (KLR riders don't carry Sat Phones right?) and call for help and give them your GPS coordinates. If the search party is using a different datum than you are... it could take a long time for anyone to find you.

Like I mentioned above, depending on where you are in the world and which datums are being used, it could just be a minor difference of only several meters, but then again, it could be significant enough that the search party can't find you (at least not quickly).

Most quality maps will state what datum was used to create the map. If you ever compare your GPS to your paper map, just make sure to go into the GPS settings and make sure it is set to the same datum as was used for the map.

When traveling in different countries you should find out what datum you should use (check local maps and/or do your research before you go to find out what is commonly used) and make sure to set your GPS unit to that datum.

If you ever have to call for help and give your coordinates, make sure to also tell them what datum you are using. If you are calling for a rescue and they don't know what datum is... I wish you the best of luck!

Clear as mud?

To see for yourself, try this little experiment:

Use your GPS to get your current lat/lon coordinates and plot them in Google Maps (which should use WGS84 by default). Go into your GPS settings and change the datum. Now get your current lat/lon coordinates and plot into Google Maps. Keep doing this for as many different datum as you want and keep plotting to Google Maps. This will show you how far off the different datums are. Just remember - don't change the datum in google maps, only on your GPS (I'm not sure if you can change datum in Google Maps, but if you can... don't). If you really want to feel lost, try this for coordinates in Hong Kong (I hear Hong Kong has the biggest difference, but I have not confirmed this).

Chances are, if you are in North America and have never changed the datum settings on your GPS unit, it will be set to WGS84 and will be fine in an emergency situation... unless search and rescue is using paper maps that haven't been updated since the 40s or 50s and they have no clue what a "datum" is.

Questions and/or comments?
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Old 10-22-2014, 09:01 AM   #5
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Default Re: GPS Users - Do You Know Your DATUM?

Most folks will never go from GPS data to map data or back. Closest most will ever get is trying to find something on your GPS from published GPS coordinates.
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Old 10-22-2014, 10:18 AM   #6
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Default Re: GPS Users - Do You Know Your DATUM?

Dat 'ummmmm confused me (sorry had to do it, as you were)
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Old 03-09-2015, 01:15 PM   #7
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Default Re: GPS Users - Do You Know Your DATUM?

Most GPS units can be configured to use any one of a long list of datums (and projections, to make things even more complicated). There is no "GPS datum". Most GPSs are set to WGS84 by default, but when you get coordinates it's important to find out what their datum (and projection) is and configure your gps with the same one. Its not really important to understand what a datum or projection is, just to know to set up your gps to match what was used to generate your coordinates.

Here's a link to geography nerd stuff to bore and confuse anyone who's interested:
https://www.maptoaster.com/maptoaster...rojection.html
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Old 03-18-2015, 04:21 PM   #8
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Default Re: GPS Users - Do You Know Your DATUM?

I used to geocache quite a lot. One of my favorites is GCGGTR, D.B. Cooper Jr. A puzzle cache, the story is that a hijacked plane took off at an airport and went at such and such a speed for so long, at which point D.B. Cooper Jr jumped out. We were tasked to find the cache by projecting the known location (the airport) by a heading and distance based on the speed x time.

The problem is that the location was projected using a Magellan GPSr, which did not take into account the curvature of the Earth like a Garmin GPSr did. A group of us hiked miles to the location we had solved and searched for an hour. No find. But we did find it on the way out, a good quarter mile away from the calculated location. Silly Magellan!

The cache container was a briefcase containing the ransom, and we only found it by spotting the melted parachute canopy; a forest fire had ravaged the area the year before.

Good times.

These days, I'd ride the KLR to the cache rather than hiking.
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Old 03-18-2015, 10:56 PM   #9
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Default Re: GPS Users - Do You Know Your DATUM?

Quote:
Most GPSs are set to WGS84
I use my handheld Garmin several times per month. I use always Latitude / Longitude. WGS84 sounds right, but I am too lazy to dig it out of my car and look at the setting right now.
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Old 10-01-2015, 01:28 PM   #10
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Default Re: GPS Users - Do You Know Your DATUM?

As a licensed Land Surveyor in 2 states I can tell you that most adv riders need not worry about datums
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Old 10-01-2015, 07:15 PM   #11
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Default Re: GPS Users - Do You Know Your DATUM?

I use decimal degrees and NAD83 because that's what I use at work. Matches up with our GIS
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Old 10-03-2015, 02:15 AM   #12
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Default Re: GPS Users - Do You Know Your DATUM?

WGS84.
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