Agisoft Metashape
Agisoft Metashape => General => Topic started by: photogrammetrix on September 15, 2013, 12:23:45 PM
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Hi everybody,
I just recently came across the question, whether PS Pro automatically takes the appropriate geodetic datum transformations into account, when conversion between coordinate reference systems requires to do so.
Example:
Photoscan project file was set up using manually picked gcp from CAD drawing which was in DHDN GK Zone 3 (EPSG:31467) projected coordinate system.
Parameters for that are (in ESRI-PRJ File compatible notation):
PROJCS["DHDN_3_Degree_Gauss_Zone_3",
GEOGCS["GCS_Deutsches_Hauptdreiecksnetz",
DATUM["D_Deutsches_Hauptdreiecksnetz",
SPHEROID["Bessel_1841",6377397.155,299.1528128]],
PRIMEM["Greenwich",0.0],UNIT["Degree",0.0174532925199433]],
PROJECTION["Gauss_Kruger"],
PARAMETER["False_Easting",3500000.0],
PARAMETER["False_Northing",0.0],
PARAMETER["Central_Meridian",9.0],
PARAMETER["Scale_Factor",1.0],
PARAMETER["Latitude_Of_Origin",0.0],
UNIT["Meter",1.0]]
Task was, to convert to geographic coordinates in WGS84 (EPSG:4326), which is defined by:
GEOGCS["GCS_WGS_1984",
DATUM["D_WGS_1984",
SPHEROID["WGS_1984",6378137,298.257223563]],
PRIMEM["Greenwich",0],UNIT["Degree",0.017453292519943295]]
Depending on the location of the project area, there are different Datum transformations applicable (here as suggested by ArcGIS projection engine):
DHDN_To_WGS_1984_1 1673 Germany - West
DHDN_To_WGS_1984_2 1777 Germany - West
DHDN_To_WGS_1984_3 15869 Germany - Brandenburg, Mecklenburg-Vorpommern, Sachsen, Sachsen-Anhalt, Thueringen
DHDN_To_WGS_1984_3x 108206 Germany - South of 50°20' N
DHDN_To_WGS_1984_4_NTv2 15949 Germany - onshore
DHDN_To_WGS_1984_4x 108207 Germany - between 50°20' N and 52°20' N
DHDN_To_WGS_1984_5x 108208 Germany - North of 52°20' N
DHDN_To_WGS_1984_6x 108209 Germany - Thuringen
DHDN_To_WGS_1984_7x 108210 Germany - Sachsen
In ArcGIS projection engine these are implemented as 7-Parameter Helmert-like transformations with the following parameters (except NTv2):
Naming Code Method dx dy dz rx ry rz ds
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DHDN_To_WGS_1984_1 1673 Coordinate_Frame 582 105 414 -1.04 -0.35 3.08 8.3
DHDN_To_WGS_1984_2 1777 Position_Vector 598.1 73.7 418.2 0.202 0.045 -2.455 6.7
DHDN_To_WGS_1984_3 15869 Position_Vector 612.4 77 440.2 -0.054 0.057 -2.797 2.55
DHDN_To_WGS_1984_3x 108206 Position_Vector 597.1 71.4 412.1 0.894 0.068 -1.563 7.58
DHDN_To_WGS_1984_4x 108207 Position_Vector 584.8 67 400.3 0.105 0.013 -2.378 10.29
DHDN_To_WGS_1984_5x 108208 Position_Vector 590.5 69.5 411.6 -0.796 -0.052 -3.601 8.3
DHDN_To_WGS_1984_6x 108209 Position_Vector 599.4 72.4 419.2 -0.062 -0.022 -2.723 6.46
DHDN_To_WGS_1984_7x 108210 Position_Vector 612.4 77 440.2 -0.054 0.057 -2.797 2.55
dx = x axis translation (meters)
dy = y axis translation (meters)
dz = z axis translation (meters)
rx = x axis rotation (arc-seconds)
ry = y axis rotation (arc-seconds)
rz = z axis rotation (arc-seconds)
ds = scale difference (parts per million, ppm)
(Source: ESRIS ArcGIS Documentation, ArcGIS 9.3.1, geographic_transformations.pdf)
My question is:
How are these things handled by Photoscan PRO?
Cheers
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Hi all,
here comes an update to my question. Please see attached image.
Cheers
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Hello photogrammetrix,
Unfortunately, currently it's not possible to choose one of the other transformations that could be more accurate for specific area, and conversion between datums in PhotoScan Pro is based on the most general transformation from EPSG registry.
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Hi Alexej,
thanks for your response.
So it is good practice for the moment to check the resulting ortho carefully for positional errors, when performing a coordinate transformation that makes a datum shift necessary. Or to do the transformation in external application like ArcGIS.
I stumbled across this because exported KMZ did not fit correctly onto GE, while the one created with ArcGIS did.
Cheers
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Hello photogrammetrix,
Transformations within the same Datum in PhotoScan should not induce any difference, but foor different Datums ArcGis may have used more accurate transformation. We'll consider the option for manual transform selection, but it may take quite a time.
One of the solutions could be to convert the coordinates of control points (markers and/or cameras) to the required Datum in the external application (like ArcGis) and to use this data in PhotoScan for referencing the model in the desired coordinate system.
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Hi Alexey,
yes, of course, there are several ways how to deal with it. And, yes, this is only relevant when we need to transform between different datums. Conversions such as e.g. from WGS84 geographic coords to WGS84/UTM32N metric coords are not touched by this, because they rely on the same spheroid.
Cheers