Closed Traverse to a known point advice

I like to do all my adjustments on the grid (SPCS) but this method sounds pretty good to me. Compass rule should work okay, just using the intermediate points as a check. Alternatively you could use a least squares adjustment and incorporate the intermediate station but probably won't get that much difference in result.

– Log a FastStatic base station for 3-5 hours at the start of the route. If you are observing for this amount of time for a pair of intervisible points I would refer to this as static, without going into the rabbit hole of # of receivers, post processing, etc...

– Simultaneously log 20 min FastStatic stations for initial back sight, intermediate check stations and two closing stations at the end of the route.

– Post Process logged data in Trimble Business Centre.

– Convert Grid to Ground Coordinates for all stations. Export Ground Coordinates to controller in advance of carrying out traverse. (This is something I’m not 100% on. Is this a valid option and will it allow me to carry out the traverse, at Scale Factor 1, between the GPS stations and get good misclosure?) Any time you are dealing with State Plane Coordinates you need to apply the scale factor to the field measured distances when bringing them into the software/program that you are using for the adjustment due to holding Grid Coordinates. Caveat - this will depend on what software you are using and method of adjustment.

Hi, first of all I just want to say what a fantastic forum this is. I have learned so much while browsing over the last few days.

I don't want to get off topic, but just wanted to say that I appreciate your kind words. It's nice to hear that we are accomplishing our primary goal. 🙂

Thanks for the replies so far. From further reading on the forum think the idea of converting grid to ground coordinates is not a viable option. Am I right in thinking my survey would no longer be in the project coordinate system and would be in a local system with very similar but slightly different coordinates?

If you are running all Trimble both field and office. I will say first the fast static / static for inter visible pairs make sure they are far apart like not less than 800 feet ideally a 1000. Hold as an azimuth. Set the other points along the route with static these do not have to be inter visible I would traverse on grid process base lines and use all data in the adjustment at same time. If you need to scale to ground you can and have both. Just makes sure in access you choose the correct datum and projection and you check the reduce to ellipsis so the total station data is reduced correctly and such.

There's no need to first run static, then process, then go grid to ground, then traverse, etc.

1. Gather all your data in the most efficient manner.

2. Import all to TBC. Pick a scale factor and modify local site settings to get to ground.

3. QC raw data, process and adjust GNSS, total station, and level data simultaneously in a network adjustment.

Am I right in thinking my survey would no longer be in the project coordinate system and would be in a local system with very similar but slightly different coordinates?

Not if I'm understanding your workflow correctly. The project system is whatever you tell it to be. If you process in grid, it's in grid. If you decide to go to ground, then there's your system right there.

That’s my thinking. I would probably rtk some starting pair. Set base up and start the static. While traversing and truck right along. If another crew was running levels it’s just about getting folks out of the way. Trimble makes it so easy. And if done correctly one can work in both systems. But depending on requirements and where. I may not even go to ground. Just keep project on grid. All depends on requirements though. He might be able with such a short run simply run levels some static and just rtk base and rover. To set all the extra control. Rtk done right can get very tight relative for most project requirements with the newer units. We had 2 miles of levels ran and I missed 1 point vertically next to a power pole at .04 ft all others were less I held one leveled position in middle and LSA in tbc with all data my rtk was so close to the levels it wasn’t a deal breaker. But my tolerance requirements were met beyond with the levels.

Thanks for all the great advice so far. If I leave my GPS points in Grid do I need to apply a scale factor in my Trimble Access job when I start traversing with the Total Station?

In Trimble Access choose the correct datum and state plane zone. Just like you would if doing RTK. Geoid and all. An approximate project height. Turn on the advanced geodetic thing. Oh for the life of me I can’t recall what menu in the job set up it is. Check the reduce to sea level/ell. Basically it reduces the total station corrected distance to sea level or the ellipsis. Which is correct then it can correctly project you to grid. You can check things by inversing grid ground or ellipsis from points of known value in your area. A EDM baseline is great place to start to get settings correctly. In the white papers of Trimble an old one they had the math in it. It also applies the corrected zenith angle as well if memory serves me correctly. Curvature and refraction along with temp is all necessary for good distance as well grid or ground. No need to type any scale in it will change anyway along your route based on scale and elevation factor . Just don’t use a false elevation like an assumed one. I honestly like to have lay long ellipsis heights for my starting control. What is the average in your area for grid vs ground. Here a rule of thumb is about a tenth per thousand feet don’t use zero as an elevation either if you are not at zero. Use the true nave 88 orthometric value for elevation if possible even if you start with a here position rough until you get a good position. It’s close enough. Or scale from a Topo map to get close

Thanks OleManRiver.<div>

I think I'm in the lucky position that the scale factor of my projection is very close to 1 at my survey location. (Ireland by the way, so working in Irish Transverse Mercator).

We'll log and process the GPS data before doing the traverse so will have the correct heights for the stations when traversing.

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So I assume your datum is ellipsis based so check what settings against some known points. So as you move east and west the scale changes on that projection. Then as elevation not navd88 in Ireland I am guessing. Will dictate or ellipsis height depending on the datum design that transverse Mercator is based on. So you probably need to verify that Trimble access is doing all the computations in your area correctly. Also maybe follow your local regulations and guidance. I am in the USA. So I have verified my areas . And continue to do checks especially when new releases are out. Are you working in UTM or an Ireland specific coordinate system.

As Rover83 says, you do not need to be scaling back and forth to ground/grid. Your data collector has a setting where you can set the grid system you want to work in. Feed in grid coordinates as your control, run your traverse as normal, and the dc will take care of the necessary scaling of observations. [EDIT: that is, it takes care of the necessary scaling to calculate coordinates - the raw observations remain the raw observations, unscaled]

Assuming a 2.6km strip of right of way, more or less, I'd proceed as follows:

1. Layout the proposed traverse station along the alignment to be topo'd at roughly 150-200 meter intervals. Zig zag the layout with marks alternating sides of the road. [EDIT: the design life of a project is sure to be measured in years so set substantial monuments that will be readily recoverable 5 years in the future - iron rods or something drilled into sound concrete - wood hubs or nails in asphalt are not permanent enough for this purpose, IMO]

2. Establish GPS Control Marks, suitable for the GPS base, adjacent to the beginning, middle, and end of the project but well out side of the right of way. As in perhaps up to a quarter mile outside, but while still remaining within RTK radio range of the traverse points in the right of way.

3. Tie these 3 GPS base points to the Spatial Reference system. Perhaps via static vectors to CORS or by using the on-line system that covers your area. Run static or RTK vectors between the 3 established marks. Feed these measurements into the adjustment software of your choice (apparently this is TBC in your case - I use StarNet, and have also done this in Leica Infinity). The coordinates will be grid.

4. With the GPS base on one of the 3 base marks tie as many of the traverse points as are suitable for GPS observation, and that your radio link can reach, with RTK. Repeat for each traverse point, moving the GPS base as radio range makes necessary. You can also use static observations for this if you prefer. For RTK I'd observe 90-120 seconds, then rotate the rod 180° and observe a second time. With your dc set to the appropriate grid system the resultant traverse point coordinates will also be grid.

5. Total Station traverse through all the points within the right of way. Doubled angles for sure, two sets of them better. DC still set to the appropriate grid system. Use the same point numbers for the points that you used when RTK'ing. It will be fine. Remember, you are collecting measurements to be adjusted later - not coordinates.

6 Simultaneously adjust the RTK vectors and the traverse data - in TBC or StarNet or Infinity, or whatever you use. Result is grid coordinates.

7. If you feel the need to really nail down the elevations, run levels including available local benchmarks. Amend the elevations as necessary. But if you have run your traverse well there isn't going to be a whole lot of difference. Unless the local datum disagrees with the CORS values.

8. If you must, here is where the scaling to ground may occur. But I discourage it. You can stay in grid and, with you dc set to the appropriate projection, continue to work in grid throughout the project life. The scale differences between grid and ground are trivial from the perspective of most road construction needs.

Working in ITM coord system. It's based on the TM projection. Same ellipsoid as UTM.

Thanks for all the replies and advice. Much clearer in my head now.

Thanks for the detailed breakdown. That's a huge help.