Convert NAVD88 to NGVD29 vertical datum

[Limpopo_River]

Hi,

I'm trying to convert a model with a vertical datum of NAVD88 [specifically, NAD83(HARN) /California zone 2 (ftUS) (EPSG::2871)] to a vertical datum of NGVD29 by using the "Convert" button in the Reference pane.  When I select the coordinate system [NAD27 + NGVD29 height (EPSG::7406)], I get the error message, "Unsupported vertical datum."  Any help to convert this model to NGVD29 would be appreciated.

Thanks,
Limpopo River

[Alexey Pasumansky]

Hello Limpopo_River,

It necessary to have corresponding geoid file for the vertical datum, in order to use a compound coordinate system. We have some geoids available here - http://www.agisoft.com/downloads/geoids/
But I cannot find the required one in the list (should be EPSG::5102?), do you know if the undulation grid for this geoid is available somewhere, so that we can convert it to the PhotoScan -accepted format?

[Alexey Pasumansky]

However, as I see, the coordinate systems are based on different horizontal datums, so you might have problems with the conversions, even haveing proper geoid file.

[Limpopo_River]

Thank you, Alexey!  I have GCP survey data in both NAD83/NAVD88 (using Geoid 12a) and NAD27/NGVD29.  I see you have the TIFF for Geoid 12a at the link you sent (very helpful). 

Could you please walk me through the steps for conversion or point me to the right place in the user manual or tutorials?  I'm hoping it won't require me to run the model again from the optimization through dense cloud generation, since they're the same points, just different coordinate systems.  For this project, there's a need to use both coordinate systems for comparison with historical maps using different horizontal / vertical datums, and we have survey data for the GCPs in both NAD27/NGVD29 and NAD83/NAVD88 (Geoid 12a). 

The images were aligned in WGS84 using the "reference" setting and using the embedded GPS metadata in the photos.  After alignment, I "converted" the coordinate system from WGS84 to NAD83/NAVD88 (EPSG::2871) and imported the GCPs in NAD83/NAVD88 (Geoid 12a).  (However, I didn't use the Geoid 12a file from the link you just sent me.  I hope this is ok since the surveyor already converted the data using this Geoid.)  Then I optimized and built the dense cloud.  All seems well with the error statistics for the GCPs.

To make the conversion to NAD27/NGVD29, I'm expecting that I need to put the Geoid 12a file in a specific location, and import the GCP survey data in NAD27/NGVD29,  thereby replacing the NAD83/NAVD88 survey data in the Reference pane.  Should I use the "convert" button before or after importing the GCPs in NAD27/NGVD29?  Or, instead of using "convert," do I simply change the coordinate system in the "Settings" dialogue box since I have the option of using survey data for either coordinate system?

[Limpopo_River]

Hi Alexey,

I've optimized and generated the dense point cloud, DEM, mesh, tiled model, and contours in "NAD83(HARN) / California zone 2 (ftUS)  (EPSG::2871)."  The surveyor has also provided elevations in both NAVD88 and NGVD29 vertical datums, but they provided horizontal (Northing and Easting) coordinates only in NAD83, not NAD27.

I used "Convert" in the Reference pane to convert the model generated in NAD83/NAVD88 to NAD27 by selecting "NAD27 / California zone II (EPSG::26742)."  My thinking was to do the horizontal conversion NAD83 --> NAD27 first, then manually enter the surveyor's elevations of GCPs relative to NGVD29 into the model to obtain accurate X,Y,Z coordinates in NAD27 / NGVD29.   

The resulting horizontal (X,Y) coordinates for my GCPs in NAD27 after the conversion are shifted approximately 17 feet in the X-Y plane relative to their Northing and Easting positions when converted to the same coordinate system (EPSG::26742) using ArcGIS or AutoCAD.  I'm surprised the default conversions in Photoscan gave such a different result than ArcGIS or AutoCAD, and curious what could be the reason for this. 

As a possible workaround, I'm thinking of duplicating the chunk with the model in NAD83/NAVD88, applying the Photoscan conversion from NAD83 (EPSG::2871) to NAD27 (EPSG::26742), then re-importing the GCPs using Northing and Easting coordinates from ArcGIS in NAD27 and elevations in NGVD29 provided by the surveyor.  Does this approach sound like it would work?

To resolve this difficulty for future projects, I'd like to create a custom PRJ conversion from NAD83/NAVD88 (EPSG::2871) to NAD27/NGVD29 (EPSG::26742).   Can you advise how to generate such a custom PRJ conversion and how to use it in Photoscan?

Thanks in advance for your help,
LR

[Limpopo_River]

The approach I described to convert from NAD83/NAVD88 (EPSG::2871) to NAD27/NGVD29 (EPSG::26742) in the 2nd to last par. above works, provided that you have the surveyed coordinates (northing, easting, and elevation) for the GCPs in both datums: 

As a possible workaround, I'm thinking of duplicating the chunk with the model in NAD83/NAVD88, applying the Photoscan conversion from NAD83 (EPSG::2871) to NAD27 (EPSG::26742), then re-importing the GCPs using Northing and Easting coordinates from ArcGIS in NAD27 and elevations in NGVD29 provided by the surveyor. 

After conversion and re-importing the GCP coordinates in the new datum (NAD27/NGVD29), the dense point cloud has the correct coordinates.  This conversion only re-projects the point cloud.  After the conversion, it's necessary to regenerate the mesh, DEM, orthomosaic, and contours if you want any of these products in the new coordinate system.   

LR

[Emapper]

Hi,

I was wondering if anyone had any success in locating where I could download geoid information for NAD27 + NGVD29 (EPSG::5102)? It doesn't appear to be on the Agrisoft list of geoid downloads and it seems Alexey asked the same question but was not answered. Any help would be appreciated.

Alexey, is there any way we can create a geoid file for that projection combination?

[Emapper]

As a follow up, I found the PRJ file for the projection I need, but Photoscan isn't recognizing it after a restart of the program, or if I click on the file directly. Here is the link to the PRJ file.:

http://spatialreference.org/ref/epsg/7406/

Hopefully this helps.

[Alexey Pasumansky]

In order to create the conversion between NAVD88 and NGVD29 heights it is necessary to have the undulation grid information related to NGVD29, is there any references to that are freely available on the Internet?