I am planning to give a few statistics in the near future on the results of PM2 positions on a recent real world job. Heavy canopy on most points, all were tied using a single PM2 with WAAS lock and a 1 minute minimum position average. All points were later tied using GPS/Conventional ties which yielded a list of camparison points numbering in excess of 30. But first I'll explain a few tips that I have gathered on using the PM2 in navigation mode.

First, we are more interested in our jobs in getting SPCs in feet. This requires a little tweaking. We work in a Lambert area which is supported by the PM2. You simply enter the parameters for your zone and enter 0.30480061 for the units to meters conversion (for US Survey feet). THIS MUST BE DONE FOR THE PRIMARY COORDINATE SYSTEM. Next get a position and record the lat/long and the SPC output. Go to Corpscon and enter the lat/long. Check your SPC values. When I did this I noticed a slight variation and simply changed my false northing and easting to correct it. The correction I am guessing will only work in a local area (as it is probably not a true constant offset). Once I did this the ProMark displayed very accurate autonomous positions in GRID SPC.

To relate just how accurate, this afternoon I set the ProMark up on our known office control point and let it position average for 25 minutes. At the end of 25 minutes I had a northing error of +4 feet, an easting error of -2 feet and an elevation error of -6 feet. You certainly don't need 25 min of data to do this, but I wanted a fairly long average to test my coordinate system. I will probably do this several times and perhaps weed out a little more conversion bias, or decide that what I have is good.

One test I do hope to play with (if I can drag Dad out on another experiment) is to do a manual differential session on several known points for a semi RTK test using 2 PM2's. Since it will output in feet, I am hoping for a 1 meter differential solution by having an operator with a PM2 on a known point carefully matching position averages to an operator with a PM2 on an unknown point.

Thought some might be curious,
Shawn

Maybe you got even better results than you think. Did your comparison take into account the about 1 m horizontal and 1 m vertical difference between the WGS 84 coordinates provided by WAAS and the NAD 83 (I presume) coordinates of your base?

Mr. G.

I don't quite know how the PM2 handles the WAAS corrections except that the Datum I used for the user grid was NAD83. Also because I "tweaked" the conversions from lat/long to SPC, I don't know that WGS84 vs NAD83 would make much difference. But to be honest this is all supposition on my part.

Shawn

PS thanks for the reply.

Where do you find time to do all this testing...Do you work?

Only when absolutely necessary.

To be honest, while many make hobbies of fishing, hunting, quilting, etc., my interest is grotesquely enough surveying. I enjoy it greatly, and so weekends and evenings are where I find or make the time. The thing is that these experiments sometimes result in improved work efficiency and productivity. At the very least they force me to consider concepts beyond the normal scope of operations...to push the envelope. If nothing else, this mental exercise causes me to learn more about the nuts and bolts.

Shawn

Edit: By the way, aren't all measurement attempts experiments? So in fact, I (and you for that matter) test our equipment, procedures, and knowledge everyday.
Modified By Shawn Billings on 11/5/2001 at 6:12 PM

Your alot like Phil S....and that's a for sure compliment...

TM

Have done a good 48 hr. of Tests as
well. We found the Spec. to be 80% true
at <3 Mtr.
A comment, that the Spec. is SAID to be
a 1 Sigma, and NOT 2 Sigma at 95%.
Anyone here the same?

My MAIN concern (and question to anyone)
is that the POINTS you get from the
Navigation mode (WAAS) are not easily
exported using the MapSend Steets software. Nor are point file options
available for Import into MapSend Streets for upload to the ProMark2.

The whole purpose of using the WAAS is
to really move in another direction that
DGPS, if you can live with the 10ft tollerance. (Spec.), and again, we are
talking "NAVIGATION POINTS", not the
static capabilities.

It would be great to have a more direct
ability to import say control networks for recon. work into the unit for
Navigational recovery, or say ASSET MGMT. point data for retreival.

There are a couple options, which Phil is making me aware of. Anyone else feel
this shortcomming, and what do you think? ANY OTHER IDEAS??

Trimbo,

Thanks for the compliment.

Terrain Mapping,

We did not receive the MapSends software with our units, and so have not been able to tinker with it. But I would certainly agree that easy upload/download of points should be available for the purposes you state.

At least the system is there. It will only be a matter of time before the software catches up.

Shawn

Shawn

When you perform differential positioning with GPS code data as you are doing with WAAS the coordinate system your results initially come out in is the coordinate system used for the coordinates of the stations producing the correctors. The WAAS stations are using ITRF acccording to Dave Doyle of NGS. The difference between ITRF 97 and NAD 83 can be found on the NGS web site by clicking on CORS, then Metadata, then Coordinates. The transformation is the one given from ITRF 97 to NAD 83(CORS96).

What I don't know is what the ProMark software actually does if you ask for NAD 83 coordinates. If it uses the transformation mentioned above it will indeed produce NAD 83 coordinates as output. However, much software assumes that WGS 84 and NAD 83 coordinates are identical. WGS 84 and ITRF 97 coordinate systems are very nearly the same, so if the software carries this over to also assume that ITRF 97 and NAD 83 are identical it will give the wrong answer. Note that in using WAAS code range results the WGS 84 coordinate system never comes into play. One of the things the broadcast correctors "correct" for is the difference between the coordinate system used for the broadcast satellite coordinates (which is WGS 84) and the coordinate system system in which the coordinates of the station or stations producing the coordinates is expressed (which in the case of WAAS is ITRF 97), so that the coordinates solved for initially come out referenced to the coordinate system used by the WAAS stations.

By the way you are very lucky that what you do for a living is something you enjoy so much that it is also your hobby. I have also been fortunate enough to be able, all my life, to do things for a living that I would have chosen to do anyway. It sure makes life a lot more fun.