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?
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.
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."
- 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.
