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.
@ashton I cannot help what other stuff like google maps and states decide to use. Google maps doesn’t state a datum or accuracy either. I just remember being involved before during and after hurricane katrina. And most of the missed opportunities revolved around everyone doing there own thing. Some were using UTM some nad83 lat longs some had old maps from nad27 some state plane coordinates mgrs pluggers or garmin and numbers are just numbers until someone puts an eye out. I know of specific people that might have been alive today if everyone involved with rescue efforts would have not made assumptions. It is one area that the government agencies actually did a good thing by working together to get everyone on the same system. Yes I understand someone using there phone in emergency situations reporting there position. They will read a lat long maybe degree minutes and seconds maybe decimal degrees hopefully the one rescuing makes the right assumptions of which it is. Tak a lat long and have it in both degrees minutes and seconds and decimal degrees. Plot it out assuming they both are one or the other maybe bad cell connection so you can’t hear well. Where is the chopper going to. This very thing happened during katrina and has happened in other disasters. Now you can see the brilliance in the mgrs utm or now for us in usa usng. One who takes five minutes can know how close or say accurate they are by the numbers 8 digits 10 meters a search team can cover 10 digits 1 meter 6 4 etc climbs to say a city location. I am not in Vermont and i am sure they have a good system. Not knocking it at all. But i blarge disasters people come from all over the usa.
When some federal government agency wanted a unified approach to hazardous materials, they made it possible for every federal, state, and local emergency response agency to order a free paper orange book with hazmat guide for every single emergency vehicle in the country. Now the guide is downloadable to your phone, but it's still free.
So where's my free USNG paper topo map?
I don't know how I would explain datum to the rescue folks. Probably something like: As measurements of the earth improve, our knowledge of actual locations improves. The lat/lon of the Empire State Building from a 1935 atlas won't exactly match the lat/lon from Google Earth or from a GPS, but it's likely to be close. It's not worth worrying about.
Using DeLorme, there's great educational value (and maybe some practical value) here: https://www.maptools.com/product/RS-DL-NHVT
Two or more decades ago, I bought a single such ruler for use on USGS topo maps. Seeing the angle that had to be used to interpolate longitudes illustrates the difference betweenn grid and lat/lon in a pretty telling way.
Whenever I try to explain a datum to someone I just say it's the thing you're measuring from. And then I ask them how tall they are. If they say 5'5" I say OK the floor is the datum and we're measuring to the top of your head. Voila.
