# Field Rotation Error's

If you are picking up data with the same GPS system that you are using to grade your field then there will be no compatibility problem with the data in and out. Also if you do any civil design or grading on these points there also will be no issues.

For example picking up data with Field Level 11, grading and putting back out with Field Level 11. There are no issues.

If for example you pick up points with Field Level 11 and you are grading with an Agguide system then you are going to have problems. The Trimble system picks the data up in a local coordinate system based on a point called the Master Bench. However Agguide works on individual points (in triangles) and every point is assumed to be in UTM (Universal Transverse Mercator) coordinates. As one set of data is based on a plane and the other is based on a squashed sphere there is no mathematically correct way to successfully match the data. We need to do an approximation. The smaller the area the closer our approximation is.

Also the Trimble data uses a true North while the UTM system's north varies depending where on the geoid you are relative to the center of the UTM zone. If you are near the edges the problem is worse and we get a larger data swing.

We have some routines that help out in this regard.

They are :

UTM create a Google Earth image. There is a tick box if you have data from Field Level 11.
UTM create a .gps file. If you have UTM data then there is a tick box to convert to Field Level 11.
Tools - Local to UTM: This converts all the points in Field Level 11 to UTM coordinates. Field Level 11 data needs to be in UTM. ie Masterbench is in UTM and all other data is relative to the MB.
Tools - UTM to local:  This converts UTM data to Field Level 11 compatible

These conversions from and to Trimble , UTM appear to be within agricultural tolerance's but we suggest that you also check.

At present we don't have any conversions to and from State Plane.

We suggest the following procedure. Assume we have a job in State Plane coordinates that we wish to grade with a Trimble system. There will be a swing. Depending on the application, where we are on the earth and size of the job depends on whether it will be a problem or not.

To correct do the following. You need to know two or more points in your state plane job that you know are accurate. You then need to get your Field Level 11 system out and survey these points. You calculate the respective bearings of the two sets of points and you rotate the state plane job so these points are now on the same bearing. We find that the scaling factor is negligible and can be ignored for typical agricultural land grading issues.

Here are some sample points. We have two points in state plane and they are:

p1 N 2089790.300 E 1463701.350  and   p2  N 2089591.640 E 1468982.380

We have surveyed these points with Field Level 11 and we have
p1 N  5000.0 E 5000.0 and p2 N 210280.01 E 14775.28

Calculating the bearings and distances we have:

Trimble local:      92~26'13"
State Plane:       92~09'16"

So to convert our state plane job to Trimble we need to rotate everything in the job clockwise by 92~26'13" - 92~09'16"
which is 0~16'57" clockwise.

Please be aware that all the required routines for this are contained within CDS. If you have the respective data in two jobs then:

• Join ( J command) and enter respective points - it reports bearing and distance
• do join for both jobs
• calculate difference
• open job with state plane. First do "save as" and get new job. Old job is now safe.
• points - select - select all
• points - rotate . rotate around master bench
• done

 TODO: Change the footer's content in Project Settings : HTML (CHM) Export : Setup HTML Template and Layout