Today I attempted to cut some land parcels for a new subdivision but the surveyor's drawings were not matching the cut polygons in the land information system. I trust the surveyor's plan more than my own skills at this stage, so i went about checking the coordinated geometry and the shapes. I discovered that the area of the polygons were beyond the expected tolerance, the bearings were off by about 10 degrees. I found the known distances to be roughly similar to the ones on the plan. Thus I concluded that I must be using incorrect bearings but I wasn't sure why.
I mentioned this issue to my supervisor and she made a note that the surveyor's plan was using Magnetic North instead of Grid North (The North Arrow has the initials MM with the arrow pointing up the page). This is important as any bearing on the plan is about 10 degrees off and becomes a significant issue over large distances. Some surveyors like to work in Magnetic North instead of Grid North, and as a newbie I failed to recognise the significance of the etchings on the North Arrow.
The good thing is that the adjustment is very simple to correct in ArcGIS.
Note: Remember this is only applicable if the surveyor plans indicate they are using Magnetic North instead of Grid North. This is a situational matter so apply the following only when necessary!
- Click on the
Ground To Grid Correctionbutton:
- The function allows you to make the correction in two ways:
- I have the surveyor's drawing and the existing parcel boundary. So i used the first method.
- Step 1 is to enter the bearing and distance of the known points.
- Step 2 is to click on the two known points.
- Once it is done, ArcGIS will suggest the correction, so apply the adjustment as displayed:
- Now when you enter in new bearings and distances, ArcGIS will make the correction as required. Remember to purge the correction from the toolbar when you're done as the adjustment will persist until you remove the correction!