Map Datum?

[+Guido71]

I've been looking for anything to tell me what datum I should set my GPS etrex Legend CX. I will be using it mostly for geocaching. What does the geocaching.com website use for a datum? The last couple caches I went after, my unit kept leading me in circles in the middle of an open field. Is there something else I might want to change settings on that would also help?

[Motorcycle_Mama]

Use WGS 84.

[wvnewbie]

I would like to know more about this datum thing.

 

I have a mag. 210 - should i be making some changes from the factory presets? and if yes, how?

[+Renegade Knight]

I would like to know more about this datum thing.

 

I have a mag. 210 - should i be making some changes from the factory presets? and if yes, how?

 

The Default Datum for a GPS from the Factory is typically WGS84.

 

You should not need to change anything. However if you do, WGS84 is the way to go. A magellan user will have to tell you how though.

[+EraSeek]

Put simply, all maps are drawn from a reference point. The old NAD27 had a spot in Kansas from which all other places were referenced. WSG84's reference point is Earth Centered and is universally used. If you have a map that lists NAD27 as it's datum, then you use that in your GPS to match the map. Otherwise use WGS84.

[vtmtnman]

Yup,WGS 84.When I first started Caching,I didn't realize that it was using WGS84 and I was set on something completely different.It cost me a few finds

 

Talk about feeling stupid.

 

"Why is my GPS 60 feet off all the time???"

Edited April 21, 2007 by vtmtnman

[wvnewbie]

thanks for the responses - based on the general accuracy of my 210 (I found a cache today and was within 20 feet according to my GPSr) I think I am ok.

[Xlobsterman]

What is the relationship between the NAD83 and WGS84 coordinates?

 

One of the questions most frequently asked by GPS users is: How do I transform coordinates between the frames defined by NAD83 and WGS84?

In effect, the GPS-user is trying to convert the positional coordinates that the receiver outputs or the manufacture proprietary software gives from one particular frame into the other. First of all, one should understand that the 3-D Cartesian frames to which the coordinates of the NAD83 and WGS84 refer are not identical. Their origin, axes orientation in space, and the unit of scale differ. Why? Simply, because the definitions of these two frames are based on different sets of observations, processing algorithms, and perhaps, geodetic assumptions.

 

In practice, any terrestrial frame is defined by a set of points with assigned coordinates and velocities. These parameters materialize the reference frame in question. The velocities are important to change positional coordinates from one epoch to another. Thus, it is critical to understand that if we refine the coordinates and/or the velocities of the defining points by introducing new and more accurate observations, then, the origin, orientation and scale of the frame may change, but to a small degree. That is why sequence of reference frames has been developed. The latest version of NAD83 and WGS84 frames are specifically named NAD83 (CORS96) and WGS84 (G1150).

 

Thus, when the receiver tells us that the computed coordinates using GPS observations are referred to the WGS84, they actually mean WGS84(G1150). On the other hand, if we use CORS coordinates data, or the OPUS service, the NAD83 reference frame to which these coordinates refer is actually NAD83 (CORS96).

 

Consequently, rigorously speaking, a transformation between NAD83 and WGS84 should be interpreted as a transformation between NAD83 (CORS96) and WGS84 (G1150). Transformations between older realizations of NAD83 and WGS84 are now outdated and will not be discussed here.

The question is then reduced to: How do we transform between NAD83 (CORS96) and WGS84 (G1150)?

 

In absolute positioning, the frame WGS84 (G1150) is materialized not by the coordinates of marks on the surface of the Earth - as the NAD83 (CORS96) is - but by the coordinates of points in space, namely, the 3-D location of the GPS satellites (given by their precise or broadcast satellite ephemerides). Recent investigations [True, 2004] have shown that for all practical purpose, the WGS84 (G1150) frame is identical to the International Terrestrial Reference Frame of year 2000 (ITRF2000). Thus, except for very accurate geophysical calculations (crustal dynamics; tectonic strain determination, etc.), the WGS84 (G1150) is supposed to be aligned with the ITRF2000. In another words, both have the same origin, orientation, and scale.

 

Once the WGS84 (G1150) and ITRF2000 are assumed equivalent [ ≡ is the mathematical symbol for equivalent], the question can be taken one step further and finally be formulated as: How do we transform between NAD83 (CORS96) and ITRF2000?

 

In order to do this transformation, the epoch at which the coordinates defining the frames were determined must be known. The National Geodetic Survey (NGS) provides coordinates and velocities of all CORS stations on these two frames: ITRF2000, epoch 1997.0, and NAD83 (CORS96), epoch 2002.0, for the conterminous states. The epoch 2003.0 was selected for the state of Alaska due to the displacements originated by the Denali earthquake on November 2, 2002. The value of these coordinates and velocities as well as a description of previous ITRF frames can be consulted in the following CORS Web page: http://www.ngs.noaa.gov/CORS/metadata1/.

 

Let’s assume now that we want to transform coordinates between the WGS84 (G1150) ≡ ITRF2000 and NAD83 (CORS96). The first thing that we need to keep in mind is that, in theory, the WGS84 (G1150) or ITRF2000 coordinates are the first one computed by the receiver software. However, they are originally obtained at the time of the observation, that is, the epoch corresponding to the mid-point of the observation window during the GPS data were collected. Consequently, we cannot compare coordinates from a set of observations taken today to results we determined one year ago. These two sets of coordinates must be reduced to a common epoch before the comparison is made. Let’s assume that we want to compare the coordinates we obtained today referred to the ITRF2000 to the ones at epoch 1997.0. Some receivers may be already doing this transformation from epoch to epoch and the final result may be already expressed in the frame WGS84 (G1150) ≡ ITRF2000 at epoch 1997.0. This will simplify the work involved, otherwise we should know the velocity of the GPS station due to the rotation of the tectonic plate on which is located. These velocities are not rigorously known until we have constantly monitored the point for a number of years. This is not generally the case and we must rely on geophysical models. The most common model used these days for correcting for plate rotations is called NNR-NUVEL-1A. This model provides the parameters required to correct for plate motion for all major plates (macroplates) forming the crust of the earth.

 

Assume for an instant, that the velocities of the three components vx, vy, and vz, are known in cm/year for a particular point, then, the transformation between epochs could be simply written:

x (epoch 1997.0) = x (epoch today) + vx * ( 1997.0 - epoch today)

with similar equations for y and z.

 

Once we have all observed points referred to a common frame and epoch, e.g. ITRF2000, epoch 1997.0, we will be able to compare apples to apples. Remember that this change from epoch to epoch may be already included in the software that you are using. In any case, one should know that all coordinates must refer to a common epoch.

 

Finally, we are ready to transform between WGS84 (G1150) ≡ ITRF2000 at epoch 1997.0 and NAD83 (CORS96) at epoch 2002.0.

 

In order to do this final step, the 14 transformation parameters (three shifts, three rotations, one scale and their variations with time) between the two frames are needed. The rigorous transformation is somewhat involved [soler and Snay, 2004], thus, the easiest approach is to use the software developed at NGS which is interactively available through the Internet. Its name is Horizontal Time Dependant Positioning (HTDP) and can be located at the following URL: http://www.ngs.noaa.gov/TOOLS/Htdp/Htdp.shtml.

 

In fact, HTDP [snay, 1999] has also the capability of transforming coordinates from epoch to epoch using in the process model NNR-NUVEL-1A mentioned above. This software also could provide transformations from the WGS84 (G1150), assumed identical to ITRF2000, to NAD83 (CORS96). The text included on the HTDP Web page is self explanatory and sufficiently detailed for doing all types of transformations necessary when performing GPS work.

 

Final comment. Although on November 5, 2006, the GPS satellite orbits provided by the International GNSS Service switch to frame ITRF2005, everything mentioned above is still valid. Nothing practically will change considering that the differences between ITRF2000 and ITRF2005 are smaller than the differences between WGS84 (G1150) and ITRF2000. Except for accurate geodetic investigations these differences could be assumed negligible.

 

References

1. Snay, R.A. (1999). Using HTDP software to transform spatial coordinates across time and between reference frames, Surveying and Land Information Systems, 59(1), 15-25.

http://www.ngs.noaa.gov/CORS/Articles/Using_HTDP.pdf

 

2. Soler, T. & R.A. Snay (2004). Transforming positions and velocities between the International Terrestrial Reference Frame of 2000 and North American Datum of 1983, J. Surv. Engrg., ASCE, 130(2), 49-55. http://www.ngs.noaa.gov/CORS/Articles/SolerSnayASCE.pdf

 

3. True, S.A. (2004) “Planning the future of the World Geodetic System 1984.” Proc. IEEE Position Location and Navigation Symposium, Monterey, CA, 26-29 April 2004, 10 p.

 

Further Reading

Below is a list of references that expand into the theory and practical applications of a topic as important as frame transformations for GPS users. We suggest that the persons desiring more information should start by reading and comprehend the series of articles about coordinate frames and datums written by Snay and Soler. For those interested on expanding into the theoretical intricacies of the problem a more technical set of citations are included. The reason that we are mainly concentrating on NGS publications is because all these references can be downloaded for free from the CORS Web page.

 

1. Snay, R.A. & T. Soler (1999). Part 1 - Modern Terrestrial Reference Systems. Professional Surveyor, 19(10), 32-33.

 

2. http://www.ngs.noaa.gov/CORS/Articles/Refe...tems-Part-1.pdf

 

3. Snay, R.A. & T. Soler (2000). Part 2 - The evolution of NAD83. Professional Surveyor, 20(2), 16, 18.

 

4. http://www.ngs.noaa.gov/CORS/Articles/Refe...tems-Part-2.pdf

 

5. Snay, R.A. & T. Soler (2000). Part 3 - WGS 84 and ITRS. Professional Surveyor, 20(3), 24, 26, 28.

 

6. http://www.ngs.noaa.gov/CORS/Articles/Refe...tems-Part-3.pdf

 

7. Snay, R.A. & T. Soler (2000). Part 4 - Practical considerations for accurate positioning. Professional Surveyor, 20(4), 32-34.

 

8. http://www.ngs.noaa.gov/CORS/Articles/Refe...tems-Part-4.pdf

 

9. Snay, R.A. (2003). Horizontal Time-Dependent Positioning, Professional Surveyor, 23(11), 30, 32, 34. ( http://www.ngs.noaa.gov/CORS/Articles/HTDPSnayPS.pdf )

 

10. Snay, R.A. (2003). Introducing two spatial reference frames for regions of the Pacific Ocean, Surveying and Land Information Science, 63(1), 5-12. http://www.ngs.noaa.gov/CORS/Articles/SalisSnay.pdf

 

11. Soler, T. & J. Marshall. (2003). A note on frame transformations with applications to geodetic datums, GPS Solutions, 7(1), 23-32. http://www.ngs.noaa.gov/CORS/Articles/GPSTrans2.pdf

 

12. Soler, T. & J. Marshall. (2002). Rigorous transformation of variance-covariance matrices of GPS-derived coordinates and velocities, GPS Solutions, 6(1-2), 76-90. http://www.ngs.noaa.gov/CORS/Articles/GPSTrans1.pdf

 

13. Soler, T., N.D. Weston, & H. Han. (2002). Computing NAD 83 coordinates using ITRF-derived vector components, Proc. XXII FIG International Congress, ACSM/ASPR Annual Conference, April 19-26, Washington D.C., 6 pg. http://www.ngs.noaa.gov/CORS/Articles/FIGSoler.doc

 

14. Soler, T. (2001). Densifying 3D GPS networks by accurate transformation of vector components, GPS Solutions, 4(3), 27-33. http://www.ngs.noaa.gov/CORS/Articles/Densifying3D.PDF

 

15. Soler, T. (1998). A compendium of transformation formulas useful in GPS work, Journal of Geodesy, 72(7-8), 482-490. ( http://www.ngs.noaa.gov/CORS/Articles/gpsPDF.pdf )

[+EraSeek]

Interesting article. I learned a few things. I assume you did not write it. Do you have a link to it?

Never the less, for the GPS users on this site, the degree of defintions do not make a lot of difference. The difference between NAD83 (CORS96) and WGS84 (G1150) make make a difference to surveyors but to Geocachers it is minimal. The difference between the two datums are somewhere in the range of a meter or so. The "rotation" of plate techtonics I believe is generally somewhere around 2cm/year. Not a huge variance for what we do. To give some perspective, the accuracy of your consumer GPS is no better than 3 meters. But interesting stuff.

[+EraSeek]

double post

Edited April 22, 2007 by EraSeek

[GeoidPS]

In summary, WG84 is universal and the default datum for all gpsr units. However, the NAD83 datum is used in the US and for current USGS topo maps. However.........the datum is so close to the WG84 datium that is is much more accurate then the location given by your unit. At worst the difference is within inches (at most).

[Xlobsterman]

Interesting article. I learned a few things. I assume you did not write it. Do you have a link to it?

 

 

http://forums.gpsworld.com/showthread.php?t=17

 

[+Klemmer]

In case someone gets into hunting for Benchmarks, we beat this issue up over in that forum a while back, including input from many experts. Like the last several posters have said, set it for WGS84 and forget it, uinless you are a surveyor. This even applies if you are looking for bechmarks (which are defined in NAD83). The difference is so small, forget it.

 

DO REMEMBER though, if you are comparing a Paper USGS Topo map (or similar in the rest of the world) to your GPS, that you DO need to set the GPS to the proper datum (NAD27 for USGS Topos). Don't forget to set the GPS back to WGS84 later!

[+Kewaneh & Shark]

The positional differences between NAD83 and WGS84 average less than a meter across the continent. For surveyors, using survey-grade, sub centimeter, GPS systems, it is a substantial difference that must be accounted for. For recreational GPSr users whose GPSr units have a ten meter accuracy (approximately five meters with WAAS enabled) a one meter difference in datum is relatively irrelevant. A GPSr set to NAD83 will still get you to within arm's reach of a cache whose coordinates were established in the WGS84 datum.

 

The differences between NAD27 & NAD83/WGS84 datums are however very substantial, and a GPSr set to NAD27 could be quite a distance from a NAD83/WGS84 coordinate.

 

- Kewaneh

[+cartographe]

For surveyors, using survey-grade, sub centimeter, GPS systems, it is a substantial difference that must be accounted for.

 

That kind of precision doesn't exist, even for surveyors. 25 000$ Leica units sport a precision of about 3-4 meters at their best when used alone. They get centimetric precision when used in differential mode, where a second gps located exactly over a precisely known benchmark, sends a mathematical correction to the first unit by radio. Or, by surveying simultaneously 2 points when one of the units is on a known benchmark for a certain period of time (depending on the distance in between those 2).

Edited May 1, 2007 by cartographe

[+Kewaneh & Shark]

For surveyors, using survey-grade, sub centimeter, GPS systems, it is a substantial difference that must be accounted for.

 

That kind of precision doesn't exist, even for surveyors....

 

That precision DOES exist if the system is used correctly. Maybe not on a Leica. Certainly not on a single phase system. Especially not on any GIS system. I routinely use Topcon 5800 series and R8 dual phase, base-stationed systems and easily get precisions less than sub centimeter. When the measurements are checked with more traditional theodolites and laser/EDMs, the checks are commonly within 0.01 foot (3.048mm) for horizontal measurements.

 

- Kewaneh, PLS