How to create a new EPSG for a local system

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How to create a new EPSG for a local system

Adrien Barbaste

Hello,

 

I’m doing an internship for an archaeology school in Greece and I have a problem in creating of a new EPSG for local coordinates.

 

I use Proj4js to work on a WebGIS and I try to determine a new EPSG for the layers that are in local coordinates.

I've all the layers in a local system built by the archeologists and some of this points in the greek coordinates system too (EGSA87).

 

First, I can calculate a 2D transformation between the local coordinates and the greek coordinates. So I obtained the 5 parameters of transformation : the 3 translations (Tx Ty and Tz), the scale factor (K) and the angle of rotation (Rz).

 

Then, I don’t know how to create a new EPSG, because I tried with a transverse mercator projection as the greek projection, but I think that it’s impossible to insert a rotation in a proj4js (only with "+axis=", but I cannot choose the angle)

So, I tried to use an oblique mercator projection with "+alpha=" (the rotation angle) and "+gamma=" (the meridian convergence)

 

The results aren't good. I've an error of less than 3cm on an archaeological site when the rotation is very low, but when I try this method on another site where the rotation angle is about 33 degrees, I have an error that can reach 15-20 centimeters.

 

I’d like to know how I can build a good EPSG for the local coordinates system ?

 

Thank you for your help

 


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Re: How to create a new EPSG for a local system

Andre Joost
Hello Adrien,

Am 12.03.2015 um 14:29 schrieb Adrien Barbaste:

> I'm doing an internship for an archaeology school in Greece and I
> have a problem in creating of a new EPSG for local coordinates.
>
> I use Proj4js to work on a WebGIS and I try to determine a new EPSG
> for the layers that are in local coordinates. I've all the layers in
> a local system built by the archeologists and some of this points in
> the greek coordinates system too (EGSA87).

I guess you want a new custom CRS, not EPSG. EPSG codes are published by
the EPSG (now IOGP) commitee, and can be found at
<http://www.epsg-registry.org/>

>
> First, I can calculate a 2D transformation between the local
> coordinates and the greek coordinates. So I obtained the 5 parameters
> of transformation : the 3 translations (Tx Ty and Tz), the scale
> factor (K) and the angle of rotation (Rz).

That is only usefull for small rotations.

>
> Then, I don't know how to create a new EPSG, because I tried with a
> transverse mercator projection as the greek projection, but I think
> that it's impossible to insert a rotation in a proj4js (only with
> "+axis=", but I cannot choose the angle) So, I tried to use an
> oblique mercator projection with "+alpha=" (the rotation angle) and
> "+gamma=" (the meridian convergence)

I have given an example here:
<http://gis.stackexchange.com/questions/83861/using-customized-coordinate-system-for-archaeological-site-data>

+lat_0 and +lonc are the geographical coordinates of the point of rotation.

If you have the coordinates in the Greek System GGRS87 (EPSG:2100), you
have to obey the datum shift between that and WGS84:

+proj=tmerc +lat_0=0 +lon_0=24 +k=0.9996 +x_0=500000 +y_0=0 +ellps=GRS80
+towgs84=-199.87,74.79,246.62,0,0,0,0 +units=m +no_defs

or for geographical coordinates EPSG:4121

+proj=longlat +ellps=GRS80 +towgs84=-199.87,74.79,246.62,0,0,0,0 +no_defs

I did not use the gamma factor in my example, because the origin of my
local CRS is on the rotation point. The gamma is used to rotate back the
CRS to have north upwards.

>
> The results aren't good. I've an error of less than 3cm on an
> archaeological site when the rotation is very low, but when I try
> this method on another site where the rotation angle is about 33
> degrees, I have an error that can reach 15-20 centimeters.

How did you compute the angle? I would suggest to calculate it from all
points you have in both coordinate systems. And it might be different
from site to site. You can try to modify the k factor as well to get
better accuracy. GGRS87 uses 0.9996, my example 1.0


HTH,
André Joost


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Re: How to create a new EPSG for a local system

Stefanos Beligiannis
In reply to this post by Adrien Barbaste
Hello Adrien,

In Greece we don't use the 2D Helmert or the similarity transformation to compute coordinates between a local system and from and to EGSA87.

We use instead  a 2nd degree polynomial transformation. Try this it will cover a 50km side rectangle with less than 2-3 cm error. This method applied by Prof. Agantza to compute parameters between the old HATT system and EGSA87.

If you want more support mail me.

Brgds
Stefanos Beligiannis

 

From: Adrien Barbaste <[hidden email]>
To: "[hidden email]" <[hidden email]>
Sent: Thursday, March 12, 2015 4:29 PM
Subject: [Proj] How to create a new EPSG for a local system

Hello,
 
I’m doing an internship for an archaeology school in Greece and I have a problem in creating of a new EPSG for local coordinates.
 
I use Proj4js to work on a WebGIS and I try to determine a new EPSG for the layers that are in local coordinates.
I've all the layers in a local system built by the archeologists and some of this points in the greek coordinates system too (EGSA87).
 
First, I can calculate a 2D transformation between the local coordinates and the greek coordinates. So I obtained the 5 parameters of transformation : the 3 translations (Tx Ty and Tz), the scale factor (K) and the angle of rotation (Rz).
 
Then, I don’t know how to create a new EPSG, because I tried with a transverse mercator projection as the greek projection, but I think that it’s impossible to insert a rotation in a proj4js (only with "+axis=", but I cannot choose the angle)
So, I tried to use an oblique mercator projection with "+alpha=" (the rotation angle) and "+gamma=" (the meridian convergence)
 
The results aren't good. I've an error of less than 3cm on an archaeological site when the rotation is very low, but when I try this method on another site where the rotation angle is about 33 degrees, I have an error that can reach 15-20 centimeters.
 
I’d like to know how I can build a good EPSG for the local coordinates system ?
 
Thank you for your help
 

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[hidden email]
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