Trimble Business Center

Greetings: If this is not the place to ask this question then please redirect.

We are about to start a small 'run-of-river' hydroelectric project. I received the first drawings yesterday and noted immediately that the project is designed in two coordinate systems: UTM and a local made up system.

The UTM covers the ROW clearing, earthworks along the penstock, site roads etc. The Intake structures and the Powerhouse are designed in the local system. The total length is approximately 2km overland between the Intake, along the penstock, and down to the powerhouse.

It appears that the two systems only overlap at the limits of each section. The control I was provided with by the engineer is clustered in two groups, one at each end: a cluster at the Intake end, and a cluster at the Powerhouse end. Nothing along the penstock alignment.

Transformation parameters have been provided between UTM and Local but tests I have done converting one to the other and back are showing small residuals (+/- 5mm).

My plan is to calibrate the site as if it was two "projects". One would be the Site UTM and the other the Site Local. I would create and maintain the Intake and Powerhouse in TBC as one project, and the Penstock as another. This would keep quantities, grade control, and layout files separate and not require the field surveyor to bounce between the two systems. I would transform only coordinates that are in the "overlap" zone between each part (ie Intake To Penstock, and Penstock to Powerhouse).

My Question: does this make sense to do it this way? I want things to be as foolproof as possible. Is there any issue with control being clustered at each end?

Any other ideas would be greatly appreciated.

Many thanks in advance.

Marshall

If the design is done that way then you have little choice I would say as we don't adjust Design Data in TBC typically at least.

The spread of the control is not ideal, but it depends somewhat on what the Geoid is doing across the site as to whether or not it will matter - it is impossible to say - I am assuming that they surveyed in the control and that the ends tied in with each other so if that is the case then it may be OK unless the Geoid is doing something strange in the middle of the job where there is no control. I would probably put control in along the job and level it in and then check it from the Site Cal created from the provided control. If it all jives nicely then you are good to go, if not then you need to work that issue as it arises.

I also forwarded to one of Trimble's experts on Site Calibrations to comment

Alan

Alan: Thanks for the quick reply. Much appreciated. I put in a call to the Engineer and asked if they had control along the Penstock corridor but haven't heard yet. I am going to guess likely not as this ground was covered by Lidar for design and it is incredibly dense, coastal forest (temperate rain forest). Once we get the clearing done I'll do an OG survey down the Penstock corridor to verify what we've been given.

I have also made the assumption that the control at each end was done and tied together. Have asked to verify by the Engineer. I guess only the site cal will tell how well it was done.

Any thoughts on dividing up the project into two based on the two coordinate systems?

Thanks again,

Marshall

Alan is correct but 5mm might be as good as you get. How many digits did you get in the Transformation parameters? A site calibration on the local site area will say something. What is the ballpark elevation at the site? Let us know if you can share the data.

Terry

Terry: Thanks for getting back to me. We got the scale factor to 10 places. Elevation goes from 2.5m to 250m.

After much consideration we have decided to do two calibrations: one for structures in the local system and one for the earthworks in UTM as they were given to us.

In my data prep I will basically run two projects. That way no one in the field has to do anything that might lead to an error. The controllers will have two sites and designs appropriate to each.

Next week when we get to site and we will perform two calibrations and see what residuals we end up with. After that we'll see if I have to account for anything else.

Thanks for the reply.

Best regards,

Marshall