Box Calibration: Here I entered points 3 and 4 which are the only control points given to us in the plans.

Transcription

1 Box Calibration: Trimble machine control does not like files that are derived from State Plane Coordinates or files that contain a Geoid. Trimble representatives want everyone to do a site calibration to remove them. This method often includes setting control on the perimeter surrounding the jobsite with a data collector that has the State Plane Coordinate system and Geoid in the file and then calibrating to those control points while in another file or using a different data collector. The first data collector usually being the Surveyor hired to do the topo for design and the second usually being the contractor with machine control. There is usually a small amount of error in setting the control with GPS and then a small amount of error when shooting the control for the calibration. Not much error, but why do it this way if it is not necessary. Here is my method of going from a file in Survey Controller or Trimble Access that is setup using State Plane Coordinate system and Geoid or a coordinate system modified with a scale factor and a Geoid to a simple projection and no Geoid. Personally, I prefer to keep my file original, not calibrated. I create an Archive subfolder inside the job folder and copy the job file to the archive folder. Rename the file with cal on the end or something similar and use this file for the box calibration. Here I entered points 3 and 4 which are the only control points given to us in the plans. Then I entered points 21 and 37 which are the start and end of the alignment for the road we are working on. The elevation is not the profile elevation. It is an approximation of the average elevation for this job. The entire range of the profile elevation is between 1320 and This is the same 4 points after changing the display to WGS84.

2 Using the latitude and longitude of 21 and 37, I designed a box that would entirely contain the job. Under options, select WGS84 as the coordinate view. I used the latitude and longitude on the yellow notepad to create 6 points in the data collector. The position of them being the circled numbers. The points were entered as G1 thru G6 by latitude and longitude and using an elipsoid height of 1260 which would give an elevation of about 1356, close enough. Points G1 thru G6 will be the Geodetic or GNSS points in the Site Calibration. The points 3 and 4 help by giving you a better idea of how your inclined plane fits with the Geoid.

3 Now it is time to enter the north and east coordinates of the same points. They will be stored as C1 thru C6. The easiest way to enter the points in coordinate view or grid view is cheating a bit but it works. Set the options to grid and attempt to enter g1. You will get this warning. Click OK or hit the enter button on the keyboard. The data collector will fill in all the blanks for G1 in north-east-elevation format but don t store it yet. G1 should remain as latitude / longitude. Change the point name to C1 and then store it. Repeat this process for points C2-C6. Now I have two sets of points forming a rectangle. G1 thru G6 are Lat/Lon and points C1 thru C6 being at the same location but stored as North and East. The elevations give you an idea of how much tilt is in the Geoid in the area you are working. From C2 to C5 there is 0.18 feet difference in elevation.

4 Now it is time to pair the points creating a calibration. Go to Survey >RTK >Site Calibration and enter C1-G1, C2-G2 as shown for all six points. The C being coordinates in Grid and the G being the GNSS points. When you have all 6 pairs entered, you will see they all match perfectly. Well they should because the C points were created from the G points. Hit Apply but we are not done yet. There is still a Geoid in there. Go to Files >Properties of current job >Local Site >Key in parameters. In the Vertical Adjustment field, change it from Geoid/Inclined plane to just Inclined Plane. The dozers will thank you later. Now in Point manager, you see that the elevations of C and G points are about 96 feet different since the Geoid has been removed. Go back to Site Calibration and hit Apply again. There could probably be a step deleted here but I like to watch the data progress.

5 The vertical residue shows the difference between using a geoid and not using a geoid. If the middle points 3 and 4 had not been entered, the residuals would probably be 0 and you wouldn t have any idea how much warp the geoid had in it. Back in Point manager, you can compare the elevations of the C and G points. It coincides with the residuals above. Now you can export the.dc or job file to use for creating the machine control files. James Rowden Grapeland Texas