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Author Topic: How does PS Pro handle geodetic datum transormations?  (Read 6826 times)

photogrammetrix

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How does PS Pro handle geodetic datum transormations?
« on: September 15, 2013, 12:23:45 PM »
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
-------------------------------------------------------------------------------------------------
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

photogrammetrix

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Re: How does PS Pro handle geodetic datum transormations?
« Reply #1 on: September 16, 2013, 12:02:44 PM »
Hi all,

here comes an update to my question. Please see attached image.

Cheers

Alexey Pasumansky

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Re: How does PS Pro handle geodetic datum transormations?
« Reply #2 on: September 16, 2013, 05:17:57 PM »
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.
Best regards,
Alexey Pasumansky,
Agisoft LLC

photogrammetrix

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Re: How does PS Pro handle geodetic datum transormations?
« Reply #3 on: September 16, 2013, 05:57:22 PM »
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

Alexey Pasumansky

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Re: How does PS Pro handle geodetic datum transormations?
« Reply #4 on: September 16, 2013, 07:03:16 PM »
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.
Best regards,
Alexey Pasumansky,
Agisoft LLC

photogrammetrix

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Re: How does PS Pro handle geodetic datum transormations?
« Reply #5 on: September 16, 2013, 07:31:39 PM »
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