GPS coordinates on maps

http://upload.wikimedia.org/wi..._NAD27_and_NAD83.png

How do you tell which datum you are using?

Thanks, Mark. This shows more variance than I had read about, and observed.

Your device, or the software you loaded it into on your computer, will have a setting. Normally in Setup, or Setting. normally called Position Datum, or Coordinate Datum, from what I have seen.

Also, for ease of entry, the Coordinate Format is simplest to enter if it is Decimal Degrees, or DD.ddddd. Also a setting on your device, or in the software.

Keep in mind, though, that once you have manually entered coordinates into your device, if you change the datum, it will translate the location to the other datum. It won't keep the numbers you entered, and reposition you in the other datum. You will have to change datums, then re-enter a waypoint location.

I think it would be better to require all GPS coordinates on maps use the WGS84 datum. I'm pretty sure it is the default for all Garmin products, and probably for all GPS products.

Well,,, Everything I do for work, which involves state agencies and private survey companies, is in NAD 83. All reports we generate have to be in NAD 83. It may be a regional variation, but I don't think requiring any specific datum wold be appropriate. If we note what it is, though, with our coordinates, anyone can set their device appropriately, and that removes any uncertainty.

NAD83 and WGS84 are virtually identical. Less than a meter difference across the US. So require either NAD83 or WGS84 on the maps but don't state the datum. It will result in a lot of worry from people whose GPS is using WGS84 and the map is in NAD83, and vice versa, over something that is of no concern.

This is all Greek to me

While I agree, Mark, that we could require NAD83 or WGS 84, I think we should also require that the datum be indicated. This could catch the occasional device that is set to NAD 27, if someone has to check the datum for notation on the map.

Jim, it was all Greek to me when Neville, Tom & Pete Reigel, and Jay were discussing the effects of slow leaks on tires, and how that changes the accuracy of calibration, or whatever they were using long-hair math for. It put me to sleep a couple of nights.

My biggest concern is that opening up this datum can of worms will result in a couple people who mess around with their settings and end up changing their datum to something wrong when it was okay in the first place. I know that might seem far-fetched, but I don't think it's any less likely than someone's datum originally being be set to NAD 27 or Rome 1940.

I don't agree with dumbing-down a process, just so someone doesn't have to understand a process. If we state the datum on the map, it is up to the person reading the map to re-set their datum.

If we require a particular datum, more people will likely have to change their datum. If, as is the case for me, I have a weekly-use datum requirement of NAD 83, and we require WGS 84, I will bounce back and forth. Not a big issue for me, but why? If my maps show NAD83, the few people that may use the coordinates will just have to make sure they are in NAD83. They may already be in it, based on local governmental datum format.

Not a big deal, either way. Just looking at all the angles. (95°, from an elevation angle of 10°)

This is all sort of greek to me also. I have tried to look it up but couldn't find anything that I could understand. I do agree with Duane after reading his remarks that if you put the GPS coordinates on a map you should use the datum used (even though the few times I have put it on my maps I didn't do it put I will in the future). I wrote an email to Garmin to find out what datum my Forerunner 201 uses.
My question would be how many measurers put the coordinates on there maps. My guess is not very many do that. I use a Garmin forerunner 201 and from my research on it I do not know what datum it uses and there is nothing on the unit itself that will let you change the datum. I bought this about 10 to 12 years ago and I can not hook it up to my mac (that I bought in 2012) so I may have to upgrade my watch also. This way when I ride the course I can get an elevation chart. Or does anyone have any ideas on the elevation chart. When you use Google earth and you go over a bridge you still get a big dip because it follows the land and not the bridge.

Given the fact that consumer GPS units have an absolute position error of over 1 meter, the NAD83 and WGS84 datums can be considered identical. There is no reason to switch between the two, and measurers can use either on their maps.
I don't think it's clear at all that including the datum used for GPS coordinates on our maps will prevent more problems than it causes. So I don't think there should be a requirement to do so.

I can see the usefulness of GPS coordinates as a backup to map descriptions. I can see how they would be helpful if not necessary for locating timing points on XC courses or in an area where there are no good fixed objects nearby with which to reference.

I am impressed when I see one of Duane's maps that has all these GPS coordinates listed. I cannot say I grasp the value of them in the usual measuring and course layout situation where there are plenty of nearby fixed objects. But I am interested in understanding more.

When I am navigating is an area that is new to me, I pick up my Android, press the navigation button, and speak the street address or the street name and city. The Little Nasal Lady takes me right where I need to go. This seems simpler to me than entering GPS coordinates into the screen. However, since I am not a big GPS fan, maybe I need some enlightenment here.

Lyman, I also post a GPX file on the Web page I create for the race. This can be loaded onto a GPS, and will show the course, and the splits.

If someone is setting the course, but is having difficulty finding one location, they can use the coordinates. I don't think they would enter every split, but they may. It is just another tool they can use, if they choose.

Little Nasal Lady can't find light poles, though, so she is not much use in finding many splits.

I use GPS coordinates for mile splits on courses where there are no nearby landmarks. This is often the case on bike trails through parks and on rails-to-trails.

I keep hearing about "no nearby landmarks". Sometimes that's really true but if you're in or near woods it helps to know trees and try to tie split points to some trees that you can pick out. I think GPS coordinates are a great way to get you pretty close but 17 ft past a 4-foot diameter sycamore can pin it down. Best to get a few such references for each point. One tree could be struck by lightning or go down in a storm but maybe they won't all disappear.
Doesn't always work, I know. But it helps.

First priority, I want my RD clients to be able to read my timing points descriptions and find them without difficulty in the light of day. I try to get them to do this in advance of race day. Paint marks can wear away quickly in high-traffic areas. I want them to not rely on paint marks.

I agree with you, Bob, about landmarks. In my experience, for as many RDs and course layout folks who do not have a sense of compass directions, there are two who wouldn't know a sycamore from kudzu. Yet, I know successful races have been conducted on courses you have measured and documented with this kind of notation.

I tried this for one race on the C&O Canal (hard-packed dirt & gravel) Towpath. The organizers were lost with what I thought were clear descriptions, i.e."2 feet north of double-trunk oak adjacent to dark stones on retaining wall". So, I went back with a camera and took pictures of each timing point from 4 compass points and put them all in a Word document with full descriptive captions. The RD had color copies of this produced at some expense. She said this was the only way they could lay out the course properly...

quote:

Originally posted by Duane Russell:
Lyman, I also post a GPX file on the Web page I create for the race. This can be loaded onto a GPS, and will show the course, and the splits.

If someone is setting the course, but is having difficulty finding one location, they can use the coordinates. I don't think they would enter every split, but they may. It is just another tool they can use, if they choose.

Little Nasal Lady can't find light poles, though, so she is not much use in finding many splits.

Right you are Duane. But, I measure mostly in urban and suburban areas where there are street addresses nearby. I try to reference house or business street address numbers as much as possible. LNL takes you right there. Not that all RDs know how to use this app on their phone.

Bob I measured a course yesterday where you literally couldn't see the forest for the trees. I'd come up with "128th pine tree east of stream crossing" if if used that method.