At the PSLS conference, I orderd three Promark 2 units; not sure when I will get them, but they are ordered.

When working off of a HARN or NGS point, what is the bearing basis obtained from the GPS?

We have done solar observations for years, and still plan to on some projects, and obtain the astronomic north and the grid north, but I'm not sure what is obtained from the GPS.

This may be a simple question, but since we're just going to start using GPS, I have many of them.

Thanks for your anticipated assistance.

If you have any good books that you woudl suggest, that would help also.

Dan Barry, NY & PA LS

When you compute a GPS vector the vector can be considered as referenced to NAD 83 geodetic north. It is actually oriented relative to WGS 84, which is slightly different. But the difference is at the 0.03 arc second level, and you are unlikely to be dealing with baselines long enough that this level of difference is significant. When your vector information has been converted to station coordinates what type of azimuth you get from your software will depend upon what type of coordinates your software uses to compute the azimuth. If it uses latitude and longitude the azimuths will be NAD 83 geodetic azimuths. If the software converts the latitudes and longitudes of points to State Plane coordinates and then computes azimuths you will have NAD 83 State Plane (ie grid) azimuths.

My favorite is by Alfred Leick called GPS Satellite Surveying...It's rather basic, but a good all around book...I think I have an older edition and it's probably been updated several times..It's published by Wiley (the premier technical publisher in my opinion)...
Have another thats called "GPS for Surveyors" but it's really too basic for anyone using GPS in my opinion...
Another good one (it's at the office) is called Introduction to Geodesy and has lots of pictures (helps me alot!)..Dont' know the author but got it on Amazon.com......
My current read is Linear Algebra, Geodesy and GPS by Stang and Borre...Very heady stuff indeed and after a chapter I usually have a headache...


I have heard from many users, that the Ashtech site itself has some very good information and primers for new users...

You might think from reading all the post on ORGIE and such, that GPS is really difficult...Not so....We are breaking the rules (actually just throwing them away) on this project...99% of the time you'll get excellent results just following the manufactures recommended occupation times and methods (watch for multipath situation, ect)..

TM

I've followed MSG Brunner on many of his surveys (which used sun shots) and match him routinely within a second using GPS.....

Your in for some serious fun....

Mr. G and TM: am I understanding this correctly, that geodetic north is the same as astronomic north?

I understand the difference between astronomic north and grid north, using both in western NY and northwest PA, and the astro-rom that we use in an HP 41 will give us both results, but am unsure if I understand the difference (if any) between geodetic north and astronomic north.

Thank you for your help! Dan Barry

Dan

Welcome to the world of GPS. I was asking the same questions you were about 18 months ago. It is amazing the difference a year can make...

To answer the geodetic v. astro question, it is my understanding that the two are very near identical. Astro bearings (which we did quite frequently before getting into GPS and shortly after) are affected by gravity. Gravity is affected by mass and so is not alway "straight down" (ie near a mountain). And so the astro bearing can be warped slightly (I have heard 20 seconds in the mountains, but could not swear to it). Geodetic bearings are to near the actual axis of the earth spin (which I understand wobbles somewhat, and so it is not precise, but I sure won't notice :-))

When we use our Ashtech software, we almost exclusively work our GPS in SPC. We then use our Carlson cogo to manipulate the coordinates from SPC to our ground system or vice versa. Knowledge of SPC will be of great benefit. If you don't have a lot of experience with it, take your time and become comfortable with it.

The fact that you do Solars speaks highly of you in my opinion. Two suggestions (and Phil has said this as well) work at first on projects that you already know the answer to. And second, since you know solars, do a few GPS sessions and then do solars between them. This will verify that you understand what you are doing as well as give you confidence in your results (thus maintaining the sleep factor).

Don't be shy about asking questions. I wore this board out when we started out. Also, the search feature at the top of the page is very powerful. Many questions you may have may have already been answered.

Good Luck,
Shawn Billings
SIT Texas

Astronomic north and geodetic north are not the same. The value one gets for astronomic north at a point is a function of the direction of the force of gravity at that point, geodetic north is not. Usually the two differ by a few seconds of arc, seldom in the US will they differ by more than 10". The largest difference I know of is at the foot of the Himalayas in India where it is of the order of one minute of arc.

To convert between astronomic and geodetic north one must know a quantity known as the deflection of the vertical at a point. This quantity can be computed if one knows the variation of the force of gravity in detail in the vicinity of a point and, less accurately worldwide. The deflection can be computed also if one has determined both the geodetic and astronomic latitudes and longitudes at a point.

NGS is capable of using gravity data and computing rather accurate deflections of the vertical using gravity data, given the approximate latitude and longitude of a point.

Again, whether you would need to determine the difference between astronomic and geodetic azimuth would depend on the accuracy you were looking for and the lengths of the lines you are dealing with.

I am surprised that there is a ROM that you can use with an HP 41 to give astronomic azimuth. I would be curious as to how it derives astronomic azimuth.

Mr. G: I've been using an Astro*Rom for an HP 41 for years; no longer available. It's set up/made by Elgin, Knowles & Senne, Inc., Rolla MO and Fayette, AR, manual dated January 1986, we started using it Jan. 1989, and is entitled User Manual HP-41 Nonephermis module for Sun/Polaris/other stars. They no longer make this ROM

We have checked our resulting grid az against NYSDOT control, with excellent results. Not sure if I've answered your question, e-mail me if I haven't at dbarry@madbbs.com

Thanks for your advice and help!

Dan Barry

There is a version for the HP-48. Operation is simular to the HP-41 module. Same folks make the HP-48 module. It uses the hour angle method.

Jerry
Modified By Jerry Davis on 2/4/2002 at 4:30 PM