UTM to NED conversion

I am having GPS and SFM (Structure for motion) trajectory in UTM coordinates. I wanted to convert the both trajectory to NED (North East Down).

Method 1

In First step: I converted the data from UTM ---> Lat, Long, Height

In second step: Lat, Long, Height ----> NED.

Method 2:

As another method I took the first data from SFM in UTM coordinate as reference. Then I subtracted the first data [easting, northing, height] from all other points. So, I got the data in meters.

The same data point [easting, northing, height] again subtracted from GPS data points. The GPS data is in now meters.

The plots looks as following:

Which method is correct for conversion of data to NED??

Because method 2 data is matching what the plot should look like in UTM!!

Whereas method 1 might have precision problem due to two conversions, that's why the both data are close and seems overlapping each other. But that is not the case with original data.

Why these both methods differ so much??

Can somebody please suggest the best method to convert UTM to NED frame??

[schrodinbug]

I'm trying to figure this now myself. How did you convert between UTM and NED? I believe that only when the origin of the NED frame is on the central meridian of the UTM zone are the grids of the NED frame and the UTM zone actually aligned. otherwise I think they're rotated slightly relative to each other. I believe rotating the NED frame by the grid convergence at the origin of the NED frame will align the frames so you can do a direct translation between them.

Answer 1

[KSSV]

UTM to degrees this might help you:

https://in.mathworks.com/matlabcentral/fileexchange/45699-ll2utm-and-utm2ll

[Ravindra]

Thank you for quick reply.

As, I have already mentioned. I already converted data from UTM ---> LLH and then LLH ---> NED.

This solution looks overlapped and considerably differ from the UTM plot. Is it because of the conversions?

Secondly When I just subtract the data in UTM frame to obtain measurement in meters the trajectory matches to the UTM plot. But, The method 1 and method 2 differ when plotted together.

The first method is followed using coordinate transformations. Is the second method correct?? If yes, the trajectory is different!

Thank you again for your help!!