I want to rent 3 Locus units to "try-before-we-buy" and set 10 control points down a relatively straight highway in 1 day.
I know what I SHOULD do, and that is, leave one on a base station and leapfrog the other two.
Trouble is, WHY this method? I have read all the posts about this technique but something still nags at me!

1/ Why don’t I first check 2 base stations by occupying them FIRST, with the third rover starting on a NEW point at the same time, then start leapfrogging after I had checked the control? (I can see danger in only using one base station. What if I wrote the value down wrong?)

2/ I know it is a stronger network if there are ties between the stations if the two units are on the NEW points simultaneously. (Using the leapfrog method)
However, if each observation takes, say, 30 minutes, then the “overlap” might only be 15-20 minutes that the rovers are actually number crunching together, before I leapfrog one of them.

3/ What about creating a Geoid model like the Leica “one step” and the Trimble “inclined plane”, surely this would make the heights more reliable instead of relying on the local Geoid model.

Locus wizards out there, what do you think? Don’t forget, you are NOT talking to an expert!

John,

First, I think what you are suggesting (as far as 10 control stations in one day) is possible. The logistics might be tough, though. Much of your time can be spent in travel from point to point, so the distance you travel will have a great impact on your success from a time standpoint. Depending on the type of control being run, we will often times establish a local control point from a fixed known and then use the one or two local control points to "densify" the rest of the project control. This will not work if you want to make your own local geoid in which case you will need 3-4 known vertical control stations which enclose your project.

The idea behind leap frogging is that you do have in essence 2 base stations. As an example lets say you have 5 points you want to tie and you have 3 rec. You would set up rec A on point 1, B on point 2, and C on point 3. (assuming point 1 is a known fixed control point) You would then "cook" until the reciever informs you that you have occupied for a long enough time to get a good vector. Then change the HI's on rec B at 2 and rec C at 3, and move rec A to point 4. You are then cooking from 2 (which is now a known) to 4 and 3 (which is now a known) to 4. after you cook that observation long enough, you would then move rec B to 5 and cook from 3 (now a known) to 5 and from 4 (now a known) to 5. You then do this until all points are tied.

As far as making your own geoid, we have only done this once using Ver 1.2. The trick is (and I may be wrong, so some one correct me) to use ellipsoid heights in your settings but input your ortho heights. Then anytime you read XXX ellipsoid it is actually XXX orthometric. Makes a mess of your reports though, and we are fortunate to be in an area in which the geoid model is very accurate so we have not messed with it much. Perhaps some one else can enlighten us both :-).

Hope this helps. I am by no means an expert at this either, but read this board to the beginning and you likely be ready to go when you get your equipment.

One other thing and I am sure you are probably already aware of this. Have all your sites ready before you go out to observe, pay attention to the DOP charts for your observation day as this will likely determine when the lunch break is, and when to make long trips to control (like having to hike in to a remote point) and cook longer than you think you need.

Shawn Billings
SIT Texas

Shawn

Great post.
From reading your method, does that mean that points 2 and 3 have recievers sitting on them for two lots of observations, and the same following down the line? I was wondering if this would probably slow things down a bit.

Maybe 4 Locus, with two on base stations and the other two leapfrogging?

Hi John,

I did an airfield taxiway site that was 2.5 miles long, straight and wide open. I used three Locus receivers with two receivers as base stations on known points and did static on 10 stations down the taxiway in more or less a straight line, in one day. I used two base stations in case any vectors failed and to get a better position on the intermediate stations as well as a check on the base points. The whole thing worked exceptionally well. As anyone will tell you, the best way to get GPS experienced is to just start doing it and post messages on this board!!..Lots of luck.

-wat-

Modified By Bill Tellier on 6/16/2001 at 4:49 PM

Shawn, why are you changing HI's? and are you continuing to do it on subsequent stations after rec B at 2 and rec C at 3?

The method I describe is extremely slow, I confess. In fact, we seldom use it but I wanted to explain my understanding of a leap-frog method.

Jim, the reason for changing HI's is two fold. 1) It will help isolate the only two major field blunders that I know of that can be made in static GPS, that being not properly set up over the point and misreading of the HI itself. 2) You are manually forcing a new observation. This helps because of the difficulty in using the TIME TRIM feature in Locus Processor V1.2. If any observation is weaker than another (eg 1st and 2nd or 1st and 3rd) then the seperate observations will help to isolate the weaker of the observations (if any weaknesses exist).

This method assures 2 observations of each point and 3 vectors (if I am figuring this correctly in my head) along with repeat vectors to every point. This is a great deal of redundancy. The method outlined by WAT is generally what we do, but not knowing how far your control points are from each other I can't say it will be the most suitable. That's a decision you must make for your situation. The leap frogging I outlined allows the maximum distance of 20k between each control pair. One more aside is that if you are not familiar with setting up a GPS network is to recognize that while there are some similarities with an optically derived control network, there are also differences. The main difference that comes to my mind that has no effect on a GPS network that you would normally be concearned with in a conventional one is the geometry of the control points on the ground. For instance site 1 and 2 could be 2000' apart and sites 2 and 3 could only be 300' apart and the numbers you get are still just as strong. Likewise, sites 1, 2, and 3 could be in or near a straight line and still be perfectly suited for a GPS network. The geometry you should be most concearned with is in the sky. This is expressed as a PDOP or GDOP and should be carefully planned around. This analogy might make the point clearer. Each reciever is actually like a prism. You have 5-8 total stations in orbit (the satellites) which are measuring to each prism and then in post processing, an inverse is made between the measurements to each prism. This concept is a huge leap from conventional, which a seasoned GPS operator will recognize but I mention it anyway as I do not know what knowledge level you have in doing a static survey. Again let me add that I am no expert but hope that what I have said will give you some food for thought. As I said before, dig through the archives on this board and you'll at least feel some comfort with the system before you ever get it.

Happy 'veyin'
Shawn Billings
SIT Texas