Explaining datum to a search team
I went to a training session for a local ambulance squad that is starting up a light search and rescue team. They want to do rescues in their immediate area that just involve guiding lost people out of the woods, or carrying injured people out of the woods. No high-angle or through-the-ice rescues. I’m a member of Civil Air Patrol and went so I’d be aware of what nearby agencies were doing.
One of the folks asked about datums, and was under the impression that all the stuff printed in the margin of a map, such as latitude, longitude, scale, etc. was the datum. I tried to explain, off the cuff, that a datum was the system used to decide the exact latitude and longitude of any point on the ground, but I’m not sure I made it clear.
So, does anyone know of a simple explanation of what a datum is, that only emphasizes the differences that people looking for a person with a hiking GPS will care about?
Also, is a term that collectively includes most of the stuff you find in the margins of a map?
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Also, is a term that collectively includes most of the stuff you find in the margins of a map?
Marginalia
The datum identification should be included in the marginalia. I’d offer this attempt at explanation:
You have to define a coordinate system and starting point in order to be able to give coordinates to a point. As we have gotten better at measuring, it has been useful to redefine our latitude and longitude coordinate system, i.e. datum, to be more self-consistent and and accurately placed on the earth.
That said, for S&R they might benefit from knowing the conversion from NAD27 to WGS84, depending on where you are, but need not bother with differences between WGS84, NAD83(xx), and NAPGD2022 as those all fall within arms reach of each other.
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For this purpose, I think
Datum=Reference Frame [or maybe even reference system]
Some of the confusion you see might stem from the definition of datum outside of surveyor jargon. Datum just means “a piece of information”. Plural Data, multiple pieces of information.
I may be wrong but it seems to me like trying to explain datums to a novice is a futile effort. What I would emphasize the most is map reading skills, especially quad sheets and how to scale lat/long positions into a quality hand held GPS device and effectively use that device for the intended purpose. Hand held compass skills would also seem like something to stress.
When you venture into the area of datums, you are probably getting into an area where the average person is going to get overloaded with information that they will not understand especially when you speak of different coordinate systems, iterations and the various transformations from one system to another.
I would try to keep it Boy Scout hiking level simple.
“Cutting up a rubber ball” for Lat-Long and “dropping a sheet of grid paper on the ground” for coordinate and datum usually works.
Grid north, u pattern searching for the win.
@chris-bouffard I wouldn’t bring up “datum” if nobody asked. But somebody did ask. Not only that, they thought that whether lat-long was expressed as decimal degrees or degree-minutes-seconds was part of the datum.
Maybe I could just say the datum is the system the mapmaker used to decide exactly where to put the lat-long markings. If the map is before 1990 and says something about “North American Datum of 1927”, your GPS unit may disagree with the map more than usual, perhaps an additional 100 feet.
If anyone asks for details I can tell them the church steeple near the town green, OD1373, is 120 feet different in the 1927 datum compared to modern datums.
Maybe I could just say the datum is the system the mapmaker used to decide exactly where to put the lat-long markings.
That’s good. I agree with Bill. Whenever I talk datums in layperson terms, I’ll go back to the origin…
“Those latitudes and longitudes are referenced from the equator or meridian, which is our zero value, just like we call “sea level” zero elevation by convention.
Just as we can gather more information about the sea level (tide gauges, etc.) and modify or update where that zero value is, we can do the same with latitudes and longitudes. [leaving out the whole “sea level is not the same everywhere” bit because that’s just confusing]
Technology advancements and improved modelling techniques have led to multiple iterations of our global coordinate systems, which means that when discussing a latitude or longitude nowadays we have to specify what system we are talking about.“
“…people will come to love their oppression, to adore the technologies that undo their capacities to think.” -Neil Postman
@norm Actually that is a really cool video for this purpose
https://www.youtube.com/watch?v=kXTHaMY3cVk – Dave Doyle video
and from the NGS Glossary
I am curious whether your emergency folks work with the United States National Grid (USNG)? See: https://www.ngs.noaa.gov/web_services/ncat/usng-service.shtml
The system was developed to support emergency personnel and similar to the MGRS. The USNG values are included on NGS data sheets as shown here:
Also, is a term that collectively includes most of the stuff you find in the margins of a map?
With regard to quad topo sheets, I’ve heard the margins of the quads called the “collar”. It was (and may still be) possible to download quads both with and without the collar. If the collar is removed you can stitch the downloaded maps together to cover larger areas.
The info in the collar could also be called metadata.
Or, just smile & nod.
@geeoddmike The responders do not work with the USNG, but are vaguely aware of the military grid reference system (MGRS) and the UTM that it’s based on. Two of the guest instructors are from the Vermont National Guard and have experience with MGRS.
There is no use of state plane coordinates.
Michael Dennis long ago suggested this approach, and I have found this to be quite effective.
Works well even during a video meet (becoming more common than standing in a room teaching GPS).
Everyone hold up something on your desk. A pencil, coffee cup, a piece of paper. Wait till folks are holding the object.
Okay, call out how high that object is. Express in your answer units please.
…. 5 feet!
10 inches!
After a couple of answers are called out, “Good.” 5 feet above what? My desk!. Great answer.
Someone said 10 inches. What did you measure that from? “My desk”. Great answer.
Are there any more heights? People on the call start saying 55′ off the ground, and a few pipe up (5,000 feat above MSL).
I ask… was anyone wrong? No, given the “accuracy” and units called out, there are multiple answers to how high the object in your hand was.
Answers of your measures required a starting point to best describe what the height of that object was. That starting point is a datum. A vertical datum in this example.
fantastically simple explanation!
thanks for the response and resource.
I have a new tool for describing the datum!!
As for the overall discussion, it’s good that the SAR meetup is happening. sounds like there needs to be a simple agreed upon process that the ICS principles were established for to avoid confusion with and misinterpreting the procedures and process. Follow the chain of command and make sure comm is simple easily understood and followed
planning always pays off.
In that situation I could almost guarantee all they wanted to know is of everyone was setting their hand held GPS to DMM, DDD or DMS.
They probably think about surveying Datums as much as I do.
When we fish the ocean and someone says there is a hot bite at such and such coordinates. That decimal point makes a huge difference.
If it the origin and rotation or direction that matters most. I would try and give them a course on the national reference system. That set of coordinates was designed specifically for emergencies. During katrina the military was using a system and others a different system so rescue pilots were going to the wrong location. Ngs has a little cheat sheet write up on the system grid. If they are in usa . I knew of coords during that time being read off and some mistakes were made confusing mgrs with national reference system. Lat and longs caused issues as well. Nad 27 lat long is not the same as nad83 lat longs on the ground. Different origin point meads ranch etc. same with wgs84 datum. Wgs84 ellipsoid vs grs80 ellipsoid all of this can get confusing for the map reader. Teach them to know what datum and coordinate system the map is in or what they set in the handheld. There are a few slides video in ngs datums by dave Doyle you could pull out where he defines a datum in simple terms. But if they do rescue and want to work with the local authorities they probably need to get educated on the national reference system grid values and how to read them get some grid squares as part of there kit and easy to learn and they should theoretically be on same page as any local police fire departments etc. i think its exact name is United States National Grid. It is based on the utm system but katrina helped prove the issue that confusion of utm mgrs and such so they built this. I read a draft if this many years ago. Because we needed to spin up the reaction teams for this effort. Fema usgs nga nsarc and many others were part of this effort. I want to say around 2009 is when fema implemented this its even on most ngs data sheets for points. So you can help them get waypoints or whatever they are called loaded for them to check themselves in different areas. Its posted towards the bottom of a data sheet about where they begin the superseded control united states national grid spatial address. I can’t remember if its read like mgrs you have the zone like 18s then eastand north so using a grid square right and up. Even a jar head can do it. Rescue this is where you should guide them to.
@olemanriver None of the local SAR teams I’ve talked to use United States National Grid. The single most popular emergency response land navigation tool in Vermont is the DeLorme Vermont – New Hampshire road atlas. It has it’s own proprietary grid plus lat & long (datum not stated). Google Maps will find a USNG, but will not output the USNG of a chosen point. So I regard USNG as a failure.
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