Ok, I have very little experience processing gps data and I could use some help. I'm using an old Sokkia Locus system, L1 only. Can I assume coordinates on a local system and get good bearings between my points when I process them? I would like to be able to base my bearings on gps observation when i am in areas without any known monuments within range. I believe this is what we did at a previous employer but I'm really not sure. I realize I will need a base on known monument if I want to get state plane coords.
Thanks for your help!
Describe your system. i.e. how many Loci do you have?
Describe what post processing program you are using.
I began using 2 ProMark 2s and within a month bought a third. With three I did quite a bit of work before adding L1/L2 receivers, but I still use the PM2s. I began using Ashtech Solutions 2.5 then 2.6 and finally 2.7. I have GNSS Solutuions but still prefer the feel of Ashtech Solutions. I downloaded from multiple CORS, usually 3, and post processed for positions, better than assuming anything. Three receivers are more than twice as productive as two. With 2 you get 1 vector at a time, with 3 you get 3.
With GNSS Solutions there is the option of using VRS, Virtual Reference Station. Solutions can be set to automatically download the three nearest CORS, calculate a Virtual L1/L2 position in the midst of your L1 receivers. You then post process very short L1 vectors for excellent L1 positions.
Paul in PA
> Ok, I have very little experience processing gps data and I could use some help. I'm using an old Sokkia Locus system, L1 only. Can I assume coordinates on a local system and get good bearings between my points when I process them? I would like to be able to base my bearings on gps observation when i am in areas without any known monuments within range. I believe this is what we did at a previous employer but I'm really not sure. I realize I will need a base on known monument if I want to get state plane coords.
>
> Thanks for your help!
if you have the parameters from WGS84-> to you local system then you don't even need to occupy a known point to get a bearing/azimuth value.
You just set up the 2 units & gather data for 30-60 minutes @1 sec.
From the processing software just set up 1 unit as base & accept the default WGS coordinates & process.
If vector is fixed then convert the 2 WGS84 coordinates to your local projected system & from there get your azimuth.
If you need precise state plane coordinates then you have to oocupy 1 point with known coordinates.
Paul,
Chris has 3 units, and is running GNSS Solutions.
Until now, we have been using his 3 Locus units in conjunction with my 2 PM3 units and my 2Hipers (L1/L2).
We now have a need for him to run a project without the use of my receivers.
Thanks,
Jimmy
I always held one GPS site fixed, called it my "Base", and moved the others around. This kept the errors from accumulating.
I used a "Quasi SPC", and then used a scale factor to put it on ground.
1/(EFxSF) is the CF
Elevation Factor (average site elevation), times the Scale factor (wherever the plane was relative to ground) divided into one.
It often looked like this:
1/(0.99992363 x 0.99997123) = Combination Scale Factor of 1.0000105149
This means that a simple translate is all that is needed, to place this job on true SPC, because the amount of change is typicly going to be in the vcty of 15' or so, and that won't mess up the relationship between the actual earth, and the "Here" position that was used.
This means that later placing this job on SPC is pretty easy, if the need arises.
Most of my jobs were done on Quasi SPC, for this very reason. I still have 6 Locus units.
N
Has he tried VRS yet?
I had tried it and found it acceptable, but since I always have L1/L2 units available have not yet tried it in my latest GNSS version.
Paul in PA
I had actually thought about that last night while driving back to the office, and then you suggested it as well. I believe you are the one that pointed out that option to me some time ago. I have L1/L2 units, so I very rarely use it.
He is going to try to collect some data today if things work out, and we will try that option.
Thanks for your suggestions.
Thanks everyone for the replies. Like Jimmy said, I've got 3 Locus units which I bought from a member of this forum and I'm using the latest GNSS Solutions.
I have not tried VRS. Since I'm still learing this I've been getting help from others in the processing.
It sounds like I will be able to do exactly what I was hoping for. The reason I'm interested in the lat/long accuracy from a single unit is that I will be needing lat/long for a cell tower site and there are no cors stations nearby. I think the required accuracy is only +-50' which I can easily get from a handheld, but I just want to be sure I'm understanding everything correctly.
Thanks again!
Yes, your bearings will be accurate if proper field procedures are used.
A modern handheld with waas will give you more accurate coords than the locus will, however, based on previous experience. Seems like I remember about 200' difference from autonomous coords to true.
I have played with the VRS option in GNSS Solutions a bit. I had someone send me a set of GPS data that were collected with Topcon HiPer Ga units and Promark2 units.
He had sent his dual frequency static files to OPUS for positions for those occupations. I took his Promark2 data and had the software create a VRS file to process against. Using that position for control, I was able to process the Promark2 data with the static files from the HiPer Ga units and was able to achieve the same positions within a couple or three hundredths for the HiPer Ga occupations as he got from the OPUS solutions. That was pretty impressive to me.
I did not know that the current download manager in GNSS Solutions would talk to a Locus receiver. If so, that prolongs the life of them yet again.
Regarding the comment above that you should collect data at a one-second interval, I don't believe that is a good idea with the Locus. The default 10-second interval should be fine for most static applications.
And to your original question about the bearings you would derive from this system. If you process in State Plane, you are going to get grid bearings. If you convert to a local coordinate system, as I understand it, you will get coordinates based on geodetic north from the point that you choose to define your local coordinate system around.
The people that I know who have done that were looking for a cardinal bearing reference for a section corner. I would prefer to use State Plane coordinates and let the software provide the convergence angle for any such corners.
Sounds like the Rx's are making you money Craig ! Glad to see they are performing as well as ever.
Two picky points for Nate:
-Murphy's law says that if you don't make the quasi SPF look different from SPF, somebody will screw up eventually because of the confusion. It is a good practice to subtract off some millions and maybe hundred thousands from the true SPC before scaling to the quasi system, and add them back after you scale anything from quasi back to true. This also has the advantage of giving more convenient numbers for work at the surface.
-The terminology used by my textbook and by Corpscon seems to be
Combined Factor = Scale Factor * Elevation Factor.
When you divide this into 1, you should call it a ground conversion factor or something else to not confuse it with Combined Factor.
> -Murphy's law says that if you don't make the quasi SPF look different from SPF, somebody will screw up eventually because of the confusion. It is a good practice to subtract off some millions and maybe hundred thousands from the true SPC before scaling to the quasi system, and add them back after you scale anything from quasi back to true. This also has the advantage of giving more convenient numbers for work at the surface.
>
This is actually the (real) law in Florida.
Back to your actual question...
The answers to your original question as to the bearings that will be derive from your GPS processing sort of got lost in the side trails above including mine. The bearing output depends on what your parameters under which you process are.
With the Ashtech Solutions or GNSS Solution software you essentially have three options if you do not count processing in UTM.
1. You can process UGS84, which will yield lat/long output that makes computing bearings between points be something most people would not attempt, including me.
2. You can process in the appropriate State Plane zone and you get grid bearings.
3. You can also process in a user-defined local coordinate system with assumed coordinates assigned to the point of your choice. In that case, the basis of bearings for the resultant positions would be geodetic north, referenced to the point you choose for your origin point.
One other further note, the Ashtech Solutions software would allow you to rotate your bearings to a pair of points of known relative position in some user-defined coordinate system. I don't know that many people ever used this capability and I did not consider it something to pursue as a general rule. Though the process of defining a local user-defined coordinate system is essentially the same in GNSS Solutions as it was in Ashtech Solutions, GNSS Solutions does not contain the capability to rotate to a pair of predefined coordinate points.
Lastly, if you are going to process to a local user-defined coordinate basis, I personally recommend processing in your appropriate State Plane zone first.
Back to your actual question...
Er..., that was supposed to say WGS84. Knew it looked wrong but couldn't spot it until it got cold.
