Static GPS Network

> Will observing on both of these benchmarks introduce a bias into our network?
>
> There will be 11 people working on this project, and we have 14 GPS receivers avaliable.
>
> We are going to run 2 sessions each day that are 4 1/2 hour sessions, with a 30 minute break between sessions.
>
> The project area is a triangle that is about 60 km east west and 60 km north south. The processing will be done in OPUS PROJECTS.

if in opus projects you are talking that many hours of data from a clear site, then you should be ok. looks to me like conditions are prime for opus DB. you might as well have them be recorded, it's not that much extra trouble

Quick answer is Yes if you hold both monuments as fixed. I would try choosing one and holding it and seeing how much difference you have between your calculated elevation and the published elevation at the second monument.

I just reread your post and you said you are processing in Opus Projects and I am not familiar with that processing, can you choose which monuments and positions to hold?

I assume there's a good reason for such long observations?

I'd process all the INDIVIDUAL opus positions.

I'd process it as a PRIVATE network, and hold ONE of the NGS marks, and let the other one float, and COMPARE.

Then, I'd assemble it all, and least square adjust it, and COMPARE how much it moved.

Your biggest problem could be an inclined plane, that does NOT match the area.

So, you simply want to compare it all, and see what the differences are, and become an expert of looking at it. I suspect that IF you do the above, that ALL or nearly all the observations will be within specs, and that by all the meandering above, you will develop confidence, and will become an actuary... that is, having a fair knowledge of all the weak spots, and how it actually fits together.

Just Nate Ramblin.....

I was on a team many years ago doing static GPS for 3D control in the Pulaski County area. We had a terrible time reconciling ellipsoid heights and elevations. We ended up running hundreds of miles of levels to try to make sense of what was happening. As I recall there are severe undulations in the geoid in that area making GPS and elevations had to combine. Obviously we were working with a primitive model and not so good software but that's a tough area.... good luck... BTW Nate is on to something IMHO

Sounds like an awesome plan of attack. Can you run any 3-wire around to any of the GPS points that are not on BMs?

Sounds like an exciting project.

Two sources of information you may wish to consider are:

1. Elevations and the Global Spatial Data Model

2. One page summary of suggested procedures

I have used the process described in item 2 quite successfully. The caveats are:

1. Using reliable control points, quality GPS observations, and proven least squares adjustment of the vectors, high quality 3-D spatial positions can be obtained.

2. Geoid modeling often (but not always) introduces uncertainty into the quality of computed elevations. Verify quality of your results using the 1-page summary. It works. Examples can be provided upon request.

Just for interests sack sake, can you provide any more information on this? The reason I ask is that I would have thought that the geoid is quite smooth in this area of the country.

Thanks!

You Can Only Hold One Point Fixed

Anything else can warp a project.

You do not indicate your project position relative to CORS.

Point you hold fixed may not neccessarily be published reference coordinates.

It is entirely reasonable to hold 1 point fixed N/E or Lat/Lon and another point held for elevation, Ellip or Ortho.

I would want to post process myself against all OPUS results. It is possible to have too many vectors in which case I would work on my project wide base and then use no more than 3 base points to process local points.

Given very favorable CORS I would select my best point or three and breakdown those 4.5 hour files into 3 to 5 OPUS-RS files. Those meaned positions would be my initial project base.

A 1/2 hour break pretty much means you are reobserving each point with the exact same piece of equipment. I believe the intent should be to get a completely different observation. Time to play "Musical Chairs GPS".

What are your plans if on "The Day" your observers show up at an already occupied control point?

BTW, final question, will this be an L1/L2 only project?

Paul in PA

> I have access to enough equipment to observe on both monuments for all of the sessions. They are only about 800 meters apart.

Although it's not stated, I infer that the 2 sessions per day will involve moving the receivers among the various stations to build the network. If that's the case, I wouldn't observe both BMs every session, unless you're trying to make work for an observer. Instead, I'd follow normal height mod procedures and make sure I get at least the minimum 2 sessions (different days, different time of day) on each of the BMs. More is better, but with 4-hour sessions getting more than 3 sessions isn't likely to strengthen the network enough to justify the expense.

The short answer on that is no. The project covers a triangular area 60 km East West and 60 km North South. The points we are setting are 15 to 20 km apart by road.

Because of travel time between stations we are doing 2 observations on the same point each day.

You Can Only Hold One Point Fixed

Paul,

OPUS Projects only works with L1 L2 data, and the data is input through OPUS Static. The project is surrounded by CORS and one of the Bench Marks is a RM for a decommissioned CORS Station, with a 1 st order elevation on it. One of the CORS Stations has a 1st order Orthometric height on the ARP.

As for finding someone else occupying the point, we plan to be on station at 0700 so they will have to get there early. If the do, we will occupy another point. I have been doing GPS for over 20 years and only one time did another surveyor show up to occupy the point we were observing. We were on station at 0800 and he showed up about 10. It was also on one of these Bench Marks. We were doing a 300 + station static network near our current project

> I'd process all the INDIVIDUAL opus positions.
>
>
> I'd process it as a PRIVATE network, and hold ONE of the NGS marks, and let the other one float, and COMPARE.
>
> Then, I'd assemble it all, and least square adjust it, and COMPARE how much it moved.
>
>
> Your biggest problem could be an inclined plane, that does NOT match the area.
>
> So, you simply want to compare it all, and see what the differences are, and become an expert of looking at it. I suspect that IF you do the above, that ALL or nearly all the observations will be within specs, and that by all the meandering above, you will develop confidence, and will become an actuary... that is, having a fair knowledge of all the weak spots, and how it actually fits together.
>
> Just Nate Ramblin.....

Nate,

To input data into OPUS Projects, you have to submitt it with OPUS Static so we will have an OPUS solution on each point.

After all of the data is submitted it will be processed with the NGS Program PAGES. Then we run it through ADJUST hold any points desired in lat long and or elevation. All of the data will be submitted to OPUS DB so it can be included in the next GEOID Model, if it is found useful. We will be observing on 6 bench marks for this project. All observations will be performed with fixed height tripods.

So You Will Have 4 Occupations On Each Point?

On a project You Post Process Yourself it is not neccessary to have all L1/L2 for good results.

Please explain how you occupy the former CORS Reference Point. Access issues?

Is it a simple as just screwing on your antenna?

Paul inPA

So You Will Have 4 Occupations On Each Point?

> On a project You Post Process Yourself it is not neccessary to have all L1/L2 for good results.
>
> Please explain how you occupy the former CORS Reference Point. Access issues?
>
> Is it a simple as just screwing on your antenna?
>
> Paul inPA

These CORS reference points are rod type monuments near the CORS Station. One is about 1 km away and the other is about 1 km west on the other.

The specifications for the project require that the data be entered into the NSRS. We can do this with OPUS DB. If OPUS Projects goes into production mode this month as planned we will follow that route. If not we will fall back to OPUS DB. We are trying for 2 to 5 cm accuracy on the elevation.