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Choosing transforms

#1

Hi,

In our ThinkGEO project we use the Proj4Projection calls to transform between projections. Like:

        // OSGB uses EPSG:27700
        // Google Earth is in a Geographic coordinate system with the wgs84 datum. (EPSG:4326)
        Proj4Projection reproject = new Proj4Projection
        {
            InternalProjectionParametersString = Proj4Projection.GetEpsgParametersString(27700),
            ExternalProjectionParametersString = Proj4Projection.GetEpsgParametersString(4326)
        };

However there are a number of different transforms that could be used to achieve this reprojection and I was wondering how we could choose which we would like to use?

For example (I believe there are a few others):
1314: https://epsg.io/1314 (Oil exploration. Accuracy better than 4m and generally better than 2m) ?
7710: https://epsg.io/7710 (Accuracy 1m)

I don’t believe 1314 or 7710 can be used as in the code snippet as they are transformations not projections.

Thanks,
Jonathan

#2

Thanks Jonathan
Yes. You can not specify the 1314 or 7710 as the parameters to get the proj4 string. But you could specify the proj4 string to the projection converter object directly. Some thing like this

Proj4Projection reproject = new Proj4Projection
{
InternalProjectionParametersString = “+proj=tmerc +lat_0=49 +lon_0=-2 +k=0.9996012717 +x_0=400000 +y_0=-100000 +ellps=airy +datum=OSGB36 +units=m +no_defs”,
…
};

But you need know the proj.4 string for the EPSG 1314 or 7710.

Thanks

Frank

#3

Hi Frank,

Thanks, but I am not sure how to get that information, or even if it is possible? My understanding may be a little off but I think:

  • ESPG:27700 and ESPG:4326 are both projections.

  • There are a number of ways to transform between those projections, with different accuracies and precision, and they tend to be suited to different areas.

  • ESPG:1314 and ESPG:7710 are two, of many, possible transformations between 27700 and 4326.

Looking on the espg.io web site they exist and have descriptions (links in original question) but nothing suitable as a projection string that I can see.

Regards,
Jonathan

#4

Thanks Jonathan,
The mapsuite provider the way to do the projection base on the proj4 parameter. For most famous projection it could be very easy to find the proj4 parameter on the epsg site. For the specify area projection we need generate the proj4. I am not sure how to genreate the proj4 string. Could you check this one see if it help.
http://help.dugeo.com/m/Insight4-0/l/200489-how-do-i-create-a-custom-coordinate-reference-system-crs

Thanks

Frank

#5

Hi Frank,

The link you sent is interesting, and maybe getting there. The following also seems relevant:

Regards,
Jonathan

#6

Thanks Jonathan,
Yes. We have to understand the Proj.4 and generate the proj.4 string.

Thanks

Frank

#7

Hi,

I don’t feel we ever really resolved this, or quite understood each other. However we have been doing something similar with ESPG:27700 and ESPG: 3857 (MapBox) and have a big discrepancy in positions. Would you like us to raise a ticket and send you that project to illustrate the problem?

Regards,
Jonathan

#8

Thanks Jonathan,
Yes. Go ahead raise a ticket and send us the project. We could look into more detail.

Thanks

Frank