GPS local.

Discussion of FieldGenius related issues and questions.

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GPS local.

Postby CHRIS L » Tue Aug 15, 2006 12:14 pm

Got a job we shot awhile ago and have about 10 points on a local grid. We now are going to work with the GPS and I need to get the GPS to transform down to the local grid we had set up previous..

2006 has changed from 2004. The PDF file on GPS trans. is still showing the 2004 screen with the control and measure point.....2006 changed those screen and now its alittle more confusing.

setting---gps trans----edit control----add control-----???

The blank for the Control points is?????? new GPS point # or the previous control point #?


I am assuming the local is punched in by hitting the local coords "select point" button and entering the??? GPS or previous shot point???

Which one goes where????

from there hit CALC para.-----then Adjust points????----then close?

From then on, will my GPS points automatically transform to the local grid from before or is there more I need to do....

Basically we need to stake about a dozen points out we calced on the local grid and want to use the GPS to get it done.

thanks guys

Chris Lazdins
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Postby ianw2 » Tue Aug 15, 2006 5:29 pm

We do this all the time. I “pre-calculate” the locations of monuments we will look for in the field. I use a grid system that starts at 20000/50000. Each of these points has numbers from 1 to 199.

We go out, find a controlling monument we are satisfied with as far as safety and security.

For example purposes, let’s assume we’ve found points 3, 5, 7 and 9 and we’re setting up on 3.

Set up the base over the point and fire up the system. We tell the DC that the base is on 3. When FG2006 asks for the measurement, I select Localize mode (It’s not in my hand so I can remember the exact wording of the screen). Record the shot as 303 (we just add 300 to the point number in the file to segregate it from the “Control Point”.

Then, we fire up the rover. It gets set up on 5, 7 and 9, consecutively. Each time, we take an RTK shot, storing it as 305, 307 and 309, respectively, in this case. Before the shot is completed, we check the box the use the point as a localization point (second box up from the bottom right of the screen). If the shot gets taken and moves to the next screen before I get chance to check the box, I hit cancel, which takes me back to the previous screen with out dumping the data (unless I hit cancel twice).

At this point, the DC want to know what to pair the current shot with as far as the control cords. Enter the number in the box, select the number from the map screen or enter the coordinates in the appropriate boxes.

When you look at the next screen, you’ll have the adjustment points displayed. They should be paired up with the “shot” points in one column and the “control points’ in the other.

Tap “Calc Parameters” to get a look at the LSA residuals for the network. As long as they look fairly intelligent, tap OK and go to the next screen.

Here, you get to look at the scale factor and the rotation from the calculated points to the control points. Unless things look really screwy, tap on “ADJUST Points” in the upper right hand corner of the screen.

You should see the DC go to work moving the lat/lon based shots to your control points.

It does take a bit of getting used to until it becomes second nature. Senor Poitras needs to revamp this portion of the manual and put together another nifty video to cover this a bit better under FG2006. However, he is a busy man! ;^)

The main source of error we find is that we are not on the points which we think we are. This leads to some very interesting scale factors well away from 1.

What I find most interesting when following the old GLO surveyors who carried their bearings forward from early morning sun shots is that the rotations are often just a few seconds. Rather amazing for the the equipment they used and the equipment I get to use today.

I hope this helps you out, Chris.

I’ll be back in the field again tomorrow doing exactly this kind of work. I’ll make sure I take better notice and notes and post again if you need it.

Cheers
Ian Wilson
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Temecula CA 92591
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Postby CHRIS L » Wed Aug 16, 2006 5:29 am

Ian

So the control point number is the existing point we shot before and the local point is the new RTK point we'll shoot today?

I know the last version used to show you the control pnt # in one column, the local pnt # next to it, and then your N,E,H deviation. With 2006 all you get is the control pnt # and then the n,e,h deviation.....no more local point shown so you have to make sure you dont screw up picking points.

After I run though that, will future shots taken with the GPS be on the control grid then?

Our main problem is we shot this with GPS, calced up the property, and came back yesturday to stake it and some yahoo must have missed the 4 bright orange lath sticking up 3' higher than the grass and mowed the entire area...ripping out the original base location...so now we have to set up a new base and go through the local transformation to line things back up. All the other point we shot are section corners right in the middle of the road so I dont think I'm about to leave a base station set up in the middle of the road.......

Thanks
Chris
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Postby ianw2 » Wed Aug 16, 2006 7:10 am

“After I run though that, will future shots taken with the GPS be on the control grid then?”

Absolutely! This is one of the biggest problems I encounter with new GPS users. They want to re-localize ever time they visit the site.

Once you have established a localization for the site DO NOT RE-LOCALIZE! There are smaller discrepancies in the location of points from day to day due to differences in the constellation geometry, perturbations in the orbits, differences in the atmosphere, etc. By re-localizing every time, you introduce these errors and they multiply.

I NEVER use RTK to collect boundary measurements. I use static occupations and then return on another day, after processing the receiver files in the office. Ten to fifteen minute static sessions give me far better quality data for the location of the boundary monuments. Then, the locations of the monuments are in SPC. With the points all on SPC, I re-localize to take out the small errors and set new monuments using RTK. I usually notice up to a half a foot of error on the first point used in the localization. Subsequent shots are usually well within tolerance of the equipment and I’m good to go.

I have encountered work performed by others who use RTK for boundary collection and don’t completely understand the localization concept. It’s not uncommon to find monuments they’ve set using RTK to be out by anything from half a foot to as much as three feet, in one case!

Well…I’m off to the field to set monuments for a small parcel. I collected the data last week using Fast Static techniques. I’ll keep track of the screens and messages and let you know how things work out. I’ll see if I can post links to the CAD work and some screen shots of the post-processed files. BTW – I’m using Topcon HiPer Lite+ with Glonass capability. I still haven’t worked out the Bluetooth issues, yet, but we are about to go wireless! It’s fantastic having just one little head at the base and one little head on the rover pole! What a difference the past fifteen years have made! Does anyone remember the old days with all the car batteries and the five or six cables to tangle up?
Ian Wilson
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Postby Brian Sloman » Wed Aug 16, 2006 7:15 am

After checking the "Use for Local Transformation" when storing the rover shot, put the Control Point number (in Ian's example, point 5, 7, or 9) in the "Control Point" field at the top of the screen. In the "Local Coordinates" section you will already see the coordinates of the shot you just took (305, 307, or 309 in Ian's example).
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Postby CHRIS L » Wed Aug 16, 2006 8:14 am

Thank Brian
Thats what I was looking for as to which number went where.



Ian

Not to doubt what you are saying about RTK. Maybe California and Illinois vary in GPS accuracy. You might have significantly more vertical difference between points there as opposed to here.....80% of Illinois is FLAT. We have shot probally 30-40 section with RTK that we have previously traveresed years back with the total station and I would say 95% of the shots were under 0.05' difference. Max we've ever had was couple tenths. We've been using RTK for 8 years now and have never found any major errors on back checks to set or found points (redundancy check the next day) Thats with a Leica 530 system.

I would love to eliminate the cables and especially the controller box the 530's use, especially since all of our data collectors are bluetooth capable. I cant seem to talk the boss into spending 25k for a new GPS system with Bluetooth.
Thanks for your help with the localization issue.
Chris
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Postby Ned Ferguson » Thu Aug 17, 2006 4:57 am

While you guys are on the subject, I have a couple of questions regarding the RTK localization. I am writing off the top of my head here, so some of my terminology may be a little off.

1. After localization, why doesn`t FieldGenius allow the user to occupy any point in the local system? It seems that you are limited to only the ability to occupy the original point that was localized on. TDS is not like this - once the localization is performed any point in the system becomes a valid control point. Aren`t the trnsformation parameters the same for all points in a small local system? I have tried this with FG and I get a message to the effect "no gps data for that point."

2. What is FG doing exactly when a scale factor is entered? Here is what I want it to do: I want the ability to occupy a point with known GPS coordinates (such as a city control monument or a previously shot point) and be on state plane rotation and GROUND scaled coordinates at the same time. In order to accomplish this, when entering a scale factor FG must use the coordinates for the control monument as the origin or centroid and scale about it. I have tried it, and FG does not do this. I find that FG "moves" the base point coordinates by around 30 feet when I enter a scale factor. This has resulted in some frustration on my part. I do not undersand it. What is FG using for the origin then? Certainly not 0,0.

I hope I am making sense here. I tried to be as clear a possible.
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Postby surveyjoe » Thu Aug 17, 2006 12:53 pm

Hi Ned,

1. FG does allow you to move the base point to a new location after localization provided the new base point was shot as an RTK point and has GPS data associated with it.

As an example, the other day I set the base unit on point 1 and localized on points 2, 3 and 4. I then shot around 200 RTK points (wetlands). When I got about 2 miles from the base, the radio started to go in and out because of dense vegetation and hilly terrain.

To finish my wetland location, I went to the top of a hill in a cow pasture near the remainder of the job and set a new RTK point number 5. I then moved the base to the new point 5 and resumed my survey.

When I moved the base to point 5, I configured the base to "Known Geodetic Location". The localization from point 1 still applies to the base set up at point 5. When I checked into known points while the base was at point 5, everything checked out to within a few hundreths.

If you move the base to a point that has not been located with RTK, you will have to establish the new base by "transform to local coordinate". This is not good and could throw everything you have already done into the ocean.

RULE OF THUMB ------- Don't move the base after localizing to any point that has not been established with RTK. Then start the base on the new RTK base point with "Known Geodetic Position". Do not re-localize for the new base point.

2. I have not tried to enter a scale factor. I always let FG2006 compute the scale factor.

Thanks

John
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Postby Jason Poitras » Fri Aug 18, 2006 2:15 pm

Hi There,

Ian and Survey Joe have done a great job of explaining how to use the localization routines. They saved me a lot of typing, thank you guys.

To answer your questions Ned:

1) In the example you give, if you want to use the same transformation parameters you need to setup the base on the original base position, or on any point in the project that was measured by GPS. When you program the base and use the "Select Position From Database" button, FieldGenius will scan the raw file and find the unadjusted GPS observation for the point you select. The base is then programmed with this position.

2) FieldGenius scales around the origin N & E defined in the transformation settings. If you localize and calculate transformation parameters, the Origin North and East will be the centroid of your measured points.
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