[CADavebert]

There was some questions and discussion related to GPS devices, and the DATUM used to provide coordinates to the Park Service for items found or noted in the park. I found some basic info from my Garmin manual that I'll post here, and then give some other info as well. I learned about what a DATUM set is the hard way, by putzing around with my GPS and setting it to something else, then not being able to find the geocache I was looking for.... trying times to say the least.

From Garmin:

Map Datums and Location Formats:
What are Map Datums?
A datum is a mathematical model of the Earth that approximates the shape of the Earth, and enables calculations to be carried out in a consistent and accurate manner. The datum is physically represented by a framework of ground monuments whose positions have been accurately measured and calculated on this reference surface. Lines of latitude and longitude on a map or chart are referenced to a specific map datum. Every chart has a map datum reference and the {GPS DEVICE} can be set to match most commonly used datum data.

If you are navigating and comparing the GPS coordinates to a map, chart, or other reference, set the map datum in the GPS unit to the same datum as the map to ensure the most accurate navigation.

Basically, the concept of latitude and longitude is much older than GPS technology. As such, there are existing maps that have latitude and longitude values on them that were calculated manually, by assigning on point as a fixed known location, then moving, measuring, and marking spots out from there. The maps are then created based on other measurements from the now known latitude and longitude lines. This means that you need to know what datum is being used to generate a set of coordinate values, otherwise you could be hundreds of feet off from the correct location.

The known points and calculations for the entire US have been set or performed a number of times. This has resulted in DATUM sets. The names of the sets usually tell us what region it is for, and when it was created. For example, NAD27 CONUS is for North America, continental US, and was created in 1927.

Many, if not all, GPS devices default to a datum set called WGS84. If you Geocache, this is the datum set that you need to use to hide and seek a cache.

According to Greg, the ranger, we should use NAD83 on our GPS devices to gather the Degree, Minute, and Second data for reporting fire ring or other information.

Also, the coordinates should be in Degree, Minute, Second format (or that's what he indicated). Most GPS devices use Degree with decimal minutes. The format Greg requested would be like (E/W)117° 23' 32.5". You can either set the GPS to show this format, or simply do some math on the decimal portion to get the number of seconds.

One thing to remember...Most GPS devices have a built in map, or you can load maps into them. They usually require you set the GPS Datum set to something specific. Be mindful of that when changing your GPS to NAD83 for the Forest Service work.

 

[mattmainster]

Thanks Dave. Here is a little more information on WGS84 in relation to NAD83 taken from

http://www.mentorsoftwareinc.com/resource/Nad83.htm

USING NAD 83 AND WGS 84 POINTS

NAD 83 and WGS 84 should be thought of" a! geographically overlapping datums (in the sense of datum as adopted coordinates). There will be points with coordinates in both datums. The action to take when confronted with two sets of coordinates for a single point is up to the user. If neither position determination contains a blunder, then the differences of coordinates should be small. In fact, the expected size of these differences can be computed from the uncertainties of the two determinations. If the differences are smaller than the accuracy required, then the user may select either determination (or some combination of the two).

"Small" differences must be properly understood here. The actual difference between coordinates may quite possibly be a meter or more. Although this might be disturbing to some, this is actually the magnitude of the uncertainty of the differences that would be computed from the uncertainties of the two coordinate determinations. It reflects the fact that the two coordinate determinations are independent and uncorrelated.



Mixing Coordinates

Surveyors are familiar with the limitations imposed when mixing the results of two independent surveys (or two datums) in a single positioning problem. Within a single survey, the relative coordinates of nearby points are much more accurate than the coordinates of either. This is not the case if the two sets of coordinates come from different surveys.

Suppose that within a local area there is both an NAD 83 point and a WGS 84 point. Suppose also that a survey is run to determine the distance between the points. The measured distance could differ from the value computed from the coordinates by a meter or more. Some might find this difference to be disturbing, but it is only a reflection of the fact that the variance of relative coordinates from two different surveys is much larger than the variance of the relative coordinates of two points from the same survey.

We thus say that the most common reason that we find differences between the NAD 83 and the WGS 84 coordinates of a point is that we are dealing with two independent determinations of the same thing. Both determinations are affected by the small statistical variations which are inherent in any measurement process. Each has its own associated standard deviation, but each is valid in its own way. The user may chose either, but must be careful about mixing coordinates.



Area of Validity

Some investigators have suggested that a difference between NAD 83 and WGS 84 is that NAD 83 is valid only within North America, while WGS 84 is valid worldwide. This is incorrect. If one has an accurate method of extending NAD 83 outside of North America, then there is no reason not to do so, nor is there any reason to think that the resulting coordinates would differ from WGS 84 coordinates. In fact, as

part of the NAD 83 adjustment, Doppler observations were used to extend the datum outside of the contiguous survey networks to isolated areas such as Greenland, Puerto Rico, and Hawaii.

Did the ranger give you reasoning for his preferences in DATUM sets? I'd be interested in hearing the reasoning. I do not think the difference in distance would be off very much... I keep my GPS set to WGS84 all the time (we use it to GeoCache also).

 

[CADavebert]

My guess is that they just try to standardize on something that is consistent between the maps they have, and the devices they use. Since the device can be changed to what the maps are, then they use the maps datum.

 

[FunJunkie]

OH My! I am confused! ???

 

[Hardluck]

The Rangers use NAD83 because all of the CA 7.5 topo maps are NAD83. Here's a link to get any topo map for CA as well as detailed aerial photos

http://casil.ucdavis.edu/casil/maps/drg/