Is this a simpler way to do Static HWY control?
Posted By STEPHEN PICKFORD on 6/23/2001 at 11:33 PM

For all you experts, this is only my wretched inexperienced opinion BUT...

I want to put 7 new control stations down a highway with 3 receivers leapfrogging. Starting at established control 1 and tying to 9.
I want to utilize ALL 3 receivers, and not have one sitting on a base station for the entire survey. In this way, there is less chance it will get stolen because they ALL are within reach while leapfrogging.

I know that the following is the best way to tie all the obs together and only requires 6 observations to complete:



(obs 1(1-2-3), obs 2(2-4-5), obs 3(3-5-7), obs 4(4-6-7), obs 5(7-8-9), and obs 6(2-6-9), This is the strongest network and is produced with the fewest observations.

BUT, to my feeble mind, HWY control is not really a network because it is usually in a straight line, and I find the (above)diagram, a very complex procedure and something I thought (after a lot of thought) could be simplified into this:

In this figure all three receivers are leapfrogged through the points, BETWEEN CONTROL STATIONS 1 and 9. The stations 1,3,5,7 are the fulcrum points that have two lots of observations. The arrowed lines are the connected lines.
obs 1(1-2-3), obs 2(3-4-5), obs 3(5-6-7) and 4(7-8-9)

THEN I do the radial shots 2, 4, 6 and 8 as obs 5(2-4-6) and obs 6(6-8-9)

It’s the same amount of observations but, I think, far easier to track and simpler to execute, especially over very long highway routes of 20 stations or more.

Experts?




Modified By STEPHEN PICKFORD on 6/24/2001 at 12:53 AM

Re: Is this a simpler way to do Static HWY control?
Posted By James Webb on 6/24/2001 at 7:50 AM

Not anywhere near an expert but if I was doing it, this may or may not be better and it only "costs" one more setup:

1> 123
2> 345
3> 567
4> 789
5> 234
6> 456
7> 678

This is using the points as shown on your second figure. Except for 1 and 9 all points have two shots at probably distinctly different times/satellite constellations.

And i think it still maintains the easy to track and execute conditions.

Lastly, "for fun", I might add 2 more setups 147 and 369(assuming the both vectors are under 20k), depending on time of day and how long the sessions have been running. Good Pdop and short sesions...no doubt I'd shoot the "extras".

just a thought,

Jimbo

EDIT; What happens to your network in the first picture when 2-9 is over 20K ?
Modified By James Webb on 6/24/2001 at 7:55 AM

Re: Is this a simpler way to do Static HWY control?
Posted By STEPHEN PICKFORD on 6/24/2001 at 8:27 AM

James, I like it!

Most of our "HWY" control doesnt go over 15kms anyway.



Re: Is this a simpler way to do Static HWY control?
Posted By James Webb on 6/24/2001 at 4:51 PM

Stephen,

Good deal. One of these other guys might have something better yet.

Jimbo


Re: Is this a simpler way to do Static HWY control?
Posted By Scott Partridge on 6/25/2001 at 2:03 PM

On ocassion when we have been faced with this type of scenario, we will put one of the receivers "off-axis" from the highway to improve the network geometry and leave it there for the entire day. It does need to be in a secure spot however. In addition, it takes a receiver out of the leap frog sequence so the network takes a bit longer to complete.


Re: Is this a simpler way to do Static HWY control?
Posted By Shawn Billings on 6/25/2001 at 6:33 PM

Scott,

I understand your "off-axis" suggestion when you do not have an adequate geoid model and must tie in vertical control, but my limited understanding of GPS is that the on the ground "geometry" is not an issue in network control. Has your experience been different?

Shawn Billings
SIT Texas


Re: Is this a simpler way to do Static HWY control?
Posted By Scott Partridge on 6/25/2001 at 8:37 PM

Geomtry can be an issue - depending upon what the required horizontal and vertical accuracy specifications are. With an adjustment, we look at the semimajor axes of the relative error ellipses on the adjusted baselines as a principal indicator.

Given a highway alignment that is essentially a straight line in a north - south direction, a network run solely along the highway will show relative error ellipses elongated in a direction more or less perpendicular to the route. Depending upon the quality of the observations, and without a receiver off-axis to tighten up the geometry in an east - west direction, our control may not meet specs.

The other issue is vertical as you alluded to. If stations are not independently reobserved at least once (a new set up), there is no check on the antenna height.

It all comes down to what the control is for and what you are contractually obligated to provide.

Of course, now we just use RTK. Slam in the points, move the base, retag everything for a redundant check and you're done. Plenty good enough for highway control points.
No muss, no fuss.




Re: Is this a simpler way to do Static HWY control? Geoid?
Posted By john stratton on 6/28/2001 at 2:34 AM

Is it possible to do what Steve is achieving, and then tie in two more control stations around (enclosing)the job and create your own geoid, or "inclined plane" for better heights.

Can you create a geoid in the Locus softawre?
Modified By john stratton on 6/28/2001 at 2:36 AM

Re: Is this a simpler way to do Static HWY control?
Posted By James Webb on 6/28/2001 at 10:32 AM

John,

I haven't covered any really large or really long projects....all under 3 mile... but YES you can create your own plane in the Locus software.

I'm in S La., relatively flat, so elsewhere it might not work. I've substituted mon. elev in the control values and held them, with very accurate results.

Also on linear projects with control at both ends I have found that "creating a line " seems to work. I've used intermediate monuments to x-check and was usually within a coupla hundreths, with long obs. times(minimum 2 blinks on the Locus receiver) These linear projects were never more than a couple hundred feet off the line between two monuments used for control.

JImbo