# Thread: Directional Drilling: True North vs. Grid North cause of error in coordinates?

1. ## Directional Drilling: True North vs. Grid North cause of error in coordinates?

Hello, I have one directional well survey from Halliburton that has map coordinates calculated out from the local grid to the state plane level, I am teaching myself how to calculate real world coordinates from this survey.

When the local grid coordinates are in the north direction all i have to do is add them the top hole state plane coordinates and my calculation matches what Halliburton calculated in state plane for each point. When the direction of the grid coordinates switches south (and I subtract this value from the top hole coordinates) I start seeing errors that gradually get larger to the end of the wellbore (.01 at start of south readings to about 7 feet at bottom). I was wondering if this could because one set of coordinates from Halliburton is in grid north and the other is in true north? Could this be a grid to ground issue? This holds true for the west directional Easting values as well (east calculations match and west don't). The information sheet says that directions and coordinates are in true north reference but im not sure if that is true for the state plane coordinates they calculated.

if this is a true north issue are there calculations I could use in Excel what will convert state plane coordinates to grid north/true north if know my grid convergence? I have not found a way to do this in arcgis.

2. ## Re: Directional Drilling: True North vs. Grid North cause of error in coordinates?

Originally Posted by aimong
Hello, I have one directional well survey from Halliburton that has map coordinates calculated out from the local grid to the state plane level, I am teaching myself how to calculate real world coordinates from this survey.

When the local grid coordinates are in the north direction all i have to do is add them the top hole state plane coordinates and my calculation matches what Halliburton calculated in state plane for each point. When the direction of the grid coordinates switches south (and I subtract this value from the top hole coordinates) I start seeing errors that gradually get larger to the end of the wellbore (.01 at start of south readings to about 7 feet at bottom). I was wondering if this could because one set of coordinates from Halliburton is in grid north and the other is in true north? Could this be a grid to ground issue? This holds true for the west directional Easting values as well (east calculations match and west don't). The information sheet says that directions and coordinates are in true north reference but im not sure if that is true for the state plane coordinates they calculated.

if this is a true north issue are there calculations I could use in Excel what will convert state plane coordinates to grid north/true north if know my grid convergence? I have not found a way to do this in arcgis.

update: grid coordinates are in true north and calculated corrdinates in grid north. will use cogo to convert to grid north for surveys that don't have coordinates calculated. I'm also looking into using a scale factor calculation instead of cogo, might be faster.

3. ## solution

Originally Posted by aimong
update: grid coordinates are in true north and calculated corrdinates in grid north. will use cogo to convert to grid north for surveys that don't have coordinates calculated. I'm also looking into using a scale factor calculation instead of cogo, might be faster.
update: created custom coordinate system and used easting and northings to adjust points to known top coordinate.