# Different results from Global Mapper and Blue Marble

jnguyen
Global Mapper UserPosts:

**3**
Hi,

Anyone please help me.

I tried to convert these numbers (x: 379328, y: 9923571 - UTM18S, WGS84) to PSAD56 (CSEG code: 24878) using GlobalMapper and Blue Marble. I hace 2 different results. In GlobalMapper, the result is 379560.57, 9923946.8. In Blue Marble, the result is 379551.83, 9923937.73. I noticed that the Blue Marble use 7 parameters, and the GlobalMapper 3 parameters and the numbers are quite different between the two. Anyway, I edited the GlobalMapper projection to be exactly the same as Blue Marble and the numbers now are 379094.665, 9923142.24.

Now, I am quite confused. Which one is right?

Thanks

Joe

Anyone please help me.

I tried to convert these numbers (x: 379328, y: 9923571 - UTM18S, WGS84) to PSAD56 (CSEG code: 24878) using GlobalMapper and Blue Marble. I hace 2 different results. In GlobalMapper, the result is 379560.57, 9923946.8. In Blue Marble, the result is 379551.83, 9923937.73. I noticed that the Blue Marble use 7 parameters, and the GlobalMapper 3 parameters and the numbers are quite different between the two. Anyway, I edited the GlobalMapper projection to be exactly the same as Blue Marble and the numbers now are 379094.665, 9923142.24.

Now, I am quite confused. Which one is right?

Thanks

Joe

## Comments

17,238I assume that you added a new custom datum with the 7-parameter transformation? The 7-parameter transforms are tricky as there are 2 standards for specifying the transformation. You may need to negate the rotation X, Y, and Z parameters when adding the transform to Global Mapper depending on which standard is used in the database where you got it. I suspect that will make it agree with Blue Marble.

Thanks,

Mike

Global Mapper Guru

gmsupport@bluemarblegeo.com

http://www.bluemarblegeo.com/

3Thanks for your quick response.

Now if I modify the Global Mapper with only 3 parameters to be the same as Blue Marble (X-axis Shift -60.31, Y: 245.935, Z: 31.008) as a new custome datum, then the new numbers are 379322.65, 9923540.03 which do not agree with those of Blue Marble.

Joe

17,238You should keep all 7 parameters as specific for the transform, just try making the 3 rotation parameters the negative of what you originally used and keep the dx, dy, dz, and scale what you had originally. You can't use the dx, dy, and dz of a 7-parameter transformation for the 3-parameter transform. That will not generate a good transformation.

Thanks,

Mike

Global Mapper Guru

gmsupport@bluemarblegeo.com

http://www.bluemarblegeo.com/

3I have solved my problem.

Joe