Pulkovo1942/Gauss_Kruger transfer to WGS84 ?

[dannyjoyride]

Hello，
I have some Soviet Union topography map(now this region belongs to Tajikistan).The projections information is Pulkovo1942/Gauss_Kruger.I want to convert them to WGS84.But I don't have the transformation parameters(I just have some GPS points coordinate of this region).I find some transformation parameters(Standard Molodensky Transformations para) on the internet,but I don't now it's accuracy,so I have some questions below.
1、Can the transformation error within one meter based on this parameters ?
2、Anybody has better method or link to find the accurate transformation parameters?

the transformation parameters link:http://surveying.wb.psu.edu/sur351/D...formations.htm

[mkennedy]

Hi,

The transformation list is originally from DMA/NIMA/NGA (National Geospatial Intelligence Agency). The Pulkovo 1942 to WGS84 entry has accuracy values of -1 for all axes of XYZ coordinates. That's not good! Basically it means the accuracy is unknown.

EPSG has a 4.5 m transformation listed at http://www.epsg-registry.org, ArcGIS supports it as Pulkovo_1942_To_WGS_1984_16. EPSG well-known ID is 15865.

A more accurate transformation usually mean local control points, and someone calculating a transformation themselves.

Melita

[dannyjoyride]

Hi,

The transformation list is originally from DMA/NIMA/NGA (National Geospatial Intelligence Agency). The Pulkovo 1942 to WGS84 entry has accuracy values of -1 for all axes of XYZ coordinates. That's not good! Basically it means the accuracy is unknown.

EPSG has a 4.5 m transformation listed at http://www.epsg-registry.org, ArcGIS supports it as Pulkovo_1942_To_WGS_1984_16. EPSG well-known ID is 15865.

A more accurate transformation usually mean local control points, and someone calculating a transformation themselves.

Melita

Thank you for your kindness. But I tried my transformation using the data of Pulkovo_1942_To_WGS_1984_16,the error is larger than 60 meters in horizontal direction. I know my georeference accuracy is poor. Is it sure that Pulkovo_1942_To_WGS_1984_16 have an accuracy within 4.5 meters ?

[mkennedy]

That transformation is for a large area, and probably does have a wide variability in its accuracy, unfortunately.

[dannyjoyride]

Thanks million for your help.
I find a strange phenomenon. In ArcMap, the on-the-fly projection is more precise than the 7-parameters projection. Then I have two questons.
1,What's the reason? So we don't need transfromation parameters in coordinate transformation because the ArcGIS system can deal with the transformation automatically?
2,I know the principle of on-the-fly projection. But what I want to konw is how the ArcGIS system do this?

[mkennedy]

ArcMap's on-the-fly projection/transformation is exactly the same as using the Project tool for vector data. With raster data, there is some checking of the current map scale which can then be used to decide how precise the raster projection/transformation needs to be (small scale means you can be less precise).

ArcMap only does one geographic/datum transformation automatically: NAD_1927_To_NAD_1983_NADCON which is for the lower 48 states of the US. If you're seeing better results in ArcMap and are not setting a transformation, I think it means the transformation is definitely not appropriate!

Melita