Can anyone shed some light on using a total station and GPS (and switching between both methods (ie grid and ground) using one job file taking into account combined scale factor and SPC coordinates? We are using a Nikon 520 and seem to be getting a few hundredths differences when shooting with TS only and shooting with TS through controller. Also, I find it aggravating that the controller takes command of the TS when connected and not allow us to shoot a distance by hitting the "meas" button on the EDM. (if I want to shoot a distance while I'm set up, I have to do it through the controller) Is this the way it's supposed to be? Can someone shed some light on the settings.
I do this often. Establish control via GPS at ground, not grid. Make a new file for your conventional methods with the total station, and transfer the control points from your GPS control file into that.
Works pretty seamless for me and take less than a minute. But I do know a couple guru's who keep it all in the same file. Personally, I don't trust it and don't speak Trimble-ese as well as them.
Oh yea, most of my work is in the 3,000 elev range, so the factors can be significant. I like things at ground.
Don't know what your controller software you're using but I do this all the time, switching back and forth GPS/Conventional using Survey Pro 5.2. Sounds like a ground vs. grid dealio with you controller software applying a scale factor to your TS measurements. You could test that theory out easily enough by switching to ground.
Just because I'm paranoid, doesn't mean they aren't out to get me.
Three methods to handle Grid/Ground:
1) Keep it on the Grid
Use GPS to determine positions of control (RTK or static) for total station survey. Use true State Plane Coordinate values (no scaling). Input a project wide combined scale factor in the data collector to apply to all collected distances (or if the collector is sophisticated enough, use the calculate combined factor at each station setting to determine the precise scale factor for each instrument set-up). Perform the total station survey (raw data will store the actual measurements while coordinates will be based on the scaled distances). End result is a coordinate file that is truly State Plane Grid.
2) Use a "Modified" Grid
In this you would use GPS to determine positions of your control (RTK or static) and scale the coordinates using a project combined scale factor applied from some origin (either a point in the project which results in smaller differences from the true grid values or from the origin which results in larger differences with but is a simpler coordinate to recall). Perform the total station survey as normal. Distances should match GPS position inverses as closely as the single combined factor will allow. If the elevation differences aren't severe, the CSF should be fairly homogeneous. If not this could be problematic.
3) Use a Low Distortion Projection
The reason State Plane or UTM produces such high scale factors in some locations is that the imaginary projection surface is so far from the actual working elevation of the project. By creating a projection that is suited to the project's elevation, combined scale factors can be mitigated to being negligible. This allow the GPS positions to closely match total station distances directly with no application of a scale factor.
Each technique has some merit. Method 1 is technically highly correct, particularly if scale factors are determined at each station. The down side to Method 1 is that your coordinates will be State Plane, which means the results of your survey will be State Plane (unless you apply some scale factor to bring the results back to something resembling the surface). So plat distances will be along some imaginary grid, acreages will be at some imaginary grid, directions will have a possibly high mapping angle, reducing their correlation to North.
Method 2 is the least technically correct, and frankly I abhor it. The coordinates no longer have any real meaning, but the distances and area will approach real surface values instead of values at some imaginary surface. If this is the approach used it's often a good idea to subtract the millions value from the coordinates so they don't have the appearance of real State Plane Coordinates. Of course, keeping good notes is also important so that you can return these pseudo SPC's to real SPC's as needed.
Method 3 is what I use daily. There is a two part article about LDP's on the American Surveyor website I wrote about this, if you're interested in knowing more about it. Using this you can have your GPS work in a 5000,5000 type system directly. With the proper software you can convert these coordinates at any time to Latitude and Longitude, State Plane, UTM or anything else. And, when properly set up, the inverses are the same as the distances you'll measure on the ground with your total station.
i apologize if this assumes too much (or not enough), but are you sure the prism offsets in the controller and the gun match?
that's the only time i've run into this issue- our scale factors around here tend to be so minimal that the only way i've ever seen significant discrepancies was when taking loooooong shots- which may have as much to do with the range of the edm as the grid/surface issue.
A simple thing to overlook is the possibility of a prism pole bubble being out of level.
That's a good way to make a $26,000 GPS system disagree with a $13,000 total station system!B-)