Our company recently purchased the ProMark 2 system and for the most part, like it very much. On a typical job, we generate two (sometimes more) control points near the site; we then assume coordinates for the control points and complete our survey with a total station.

Back in the office we process the GPS data, and based on the solutions we determined an appropriate combined scale factor to use. Then, we substitute the assumed control point coordinates with the calculated GPS coordinates in our raw data file. And finally we reprocess the raw data with the calculated combined scale factor.

Is this the best way to generate grid coordinates? I already noticed that coordinates calculated by resection or intersection are not regenerated, thus causing a big headache.

So, can anybody give me some advice on a better method?

There are several alternatives that may vary based on the way you work but let me offer a couple of ideas and you can poke at what I suggest to see if we can find the best way for you.

I am guessing that you are making some GPS measurements at the same time you do some total station work. Your total station work is based on assumed coordinates. Your GPS work will be processed and used to determine a basis of bearings for your total station work.

One alternative is to use the grid coordinates produced from the GPS work to do an inverse and simply rotate the bearings in your total station work to match the inverse from the GPS measurements. You can do this in most COGO software that I know of.

Another alternative is to compute latitudes and longitudes from the GPS work and use a program called Inverse from the NGS to determine a Geodetic bearing between two of your points and, again, use your COGO software to rotate the total station work to the bearing of your choice.

This may let you preserve the integrity of your total station work just a little better.

I am not going to talk about the problems involved in choosing two points on an assumed coordinate system as a basis for either "geodetic" or "grid" bearings. That is for a discussion in a state plane coordinate seminar or one of my Weekly Wipes.

Does this help?

Phil,

I understand and agree with what you said, but this does not convert my measured total station distances to grid, it just converts my assummed azimuth to grid azimuth.

Cory

Some more info. please.
When you say you generate control points, are you actually connecting to the SPC system, or are you using autonomous positioning of one of your units to give you an approximate position? If you are not actually tieing to the SPC system then you really have a user defined system which would be best used by not applying a combined factor, but using the grid north as basis of bearing only. Stick with your total station distances.

I am located in Canada, so we don't have what you call "State Plane", but I believe our provincial system is the equivalent meaning that our control monuments have grid coordinates. And I did tie into one of these monuments.

Cory

Cory,

How big is your project? If it's under 10 miles, (16 kilometers) use ground distances and forget the grid factor. It can hurt you more than help.

Jobo

I'm concerned about your comment.
**And finally we reprocess the raw data with the calculated combined scale factor.**

I am not aware of a place to imput the combined scale factor into the Ashtech Solutions and reprocess. The only thing close is the tab/box you put the vector scaling factor and that would not help. BR>Rb


Modified By Randy Black on 12/27/2001 at 6:08 PM

I apologize for my lack of clarity, but what I meant is the Total Station raw data, not the GPS raw data. And I am using SurvCADD to reprocess my raw data, which allows me to apply the combined scale factor to all my measured EDM distances.

I may be misunderstanding your question but:

If you are trying to get your conventional TS data into grid simply reduce each distance to grid by any of several methods. This involves a simple multiplication.

1) most simply compute a combined grid + elevation scale factor for the project area. Assuming less than 500 feet vertical change and project less than 10 or so miles in extent this will most likely be adequate.

2) if you want to be a little better you can compute the specific scale factors for each line. Elevation based on average of endpoints, average scale factors of endpoints. This can be obtained by using coordinate values unreduced to get the scale factors and elevations.

Your angles won't need any modification unless very long sites.

If you have the ability to apply a combined factor, then I don't understand what the problem is. Resection or intersections that are based on azimuths or angles will recompute in grid. A resection that includes distances will need the distance reduced to grid in the same fashion.

If you need to assess whether you should apply the first or second level on scale factors, pick extremes over the job and see how much they vary from a combined average factor in PPM using an imaginary 1000 foot distance.

- jlw

The easiest way I believe is to define your own ground definition ( in solutions ver 2.4 or greater)and then export an ascii file as a ground coordinate. Solutions will do all the scaling of your vectors/baselines for you. Then import ascii text file into carlson.
RB

Cory, I am hoping that we got you fixed up with a little extra help from the folks at Carlson. If we still do not have a solution that works for you please let me know.