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Author Topic: Google Earth for GCP no geotag photos  (Read 5808 times)

Viking69

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Google Earth for GCP no geotag photos
« on: April 27, 2017, 10:57:08 PM »
I have built a model based on non geotagged photos. How do I use coordinate data from Google Earth for GCP information and what should I place in the altitude field? I want to build a dem but do not want my model to be upside down. Can I build the model then place GCP's?

Thanks for tips on workflow!

Packy_GIS

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Re: Google Earth for GCP no geotag photos
« Reply #1 on: April 27, 2017, 11:53:53 PM »
Well - the Google DEM is based on very sparse data in some regions - I would look for some more precise data elsewhere (i. e. national LIDAR survey, or try the USGS geoeplorer and the DEM section).
Jan

Viking69

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Re: Google Earth for GCP no geotag photos
« Reply #2 on: April 28, 2017, 12:04:49 AM »
Thanks Packy for the references. Any recommendation on the workflow to do this? Ie can I build the model, then add markers, and enter the coordinates to the markers based on common points located in Google Earth or one of your recommended data sources?

I really could use some help on the step by step.

SAV

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Re: Google Earth for GCP no geotag photos
« Reply #3 on: April 28, 2017, 01:39:44 AM »
Hi Viking69,

Worldwide 90m and 30m resolution DEMs are freely available, thanks to NASA (more information at https://www2.jpl.nasa.gov/srtm/).

In the past it was quite painful to find the right tile to download, but it is much easier today.

Simply go to:
http://dwtkns.com/srtm/
OR
http://dwtkns.com/srtm30m/

and download the tile you want.

Unzip the file and open it in a GIS software package to get your z coordinate.
I suggest to use QGIS (www.qgis.org) which is free and open source. Btw, qGIS can also directly 'stream' tiles from Google Maps in case you want to see the satellite imagery.

Note: Using such 'low accuracy' ground control points will influence the quality of your model. If you can, try to insert a few scale bars (using objects with a known length) which help to properly scale the model.

I hope that helps.

Regards
SAV

« Last Edit: April 28, 2017, 01:41:45 AM by SAV »

Viking69

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Re: Google Earth for GCP no geotag photos
« Reply #4 on: April 28, 2017, 07:34:02 PM »
Thanks for the links to the data sources.

What I am asking is:

My camera does not geotag photos but I can bring them into Agisoft and go through all of the steps and build a model. The model is not referenced to any coordinate system and sometimes when I export the model as DXF and open it in Global Mapper or CAD the model is upside down. I am hoping I can add a few ground registry points from Google Earth to get a rough spatial orientation to my model. I can pick coincident locations between Google Earth and my model and when I place markers, set the coordinate system and manually enter lat long information from Google Earth and hit update but I am still running into issues with my model. I  am manually entering these fields and not importing a GCP file but have triple checked that I am not messing up the coordinate entry.  Does the elevation of the Google Eath "GCP" have to be entered in the altitude field for the markers? If I do not prescribe something in this altitude field am I still going to have issues with a Z orientation? It seems I need to be able to define which way is up, and was hoping to accomplish this by using rough GCP's (X,Y,Z) from Google Earth. 

If someone has a workflow for this it would be deeply appreciated.

SAV

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Re: Google Earth for GCP no geotag photos
« Reply #5 on: May 01, 2017, 03:12:23 AM »
Yes, the elevation you recorded in Google Earth needs to be entered in the altitude field of each marker. This should eliminate the upside-down issue. And double check the coordinate reference system, which should probably be WGS84.

Regards
SAV
« Last Edit: May 02, 2017, 03:17:32 AM by SAV »

Jeremiah_ROWE

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Re: Google Earth for GCP no geotag photos
« Reply #6 on: May 02, 2017, 04:53:35 PM »
This is recommended for getting altitude values for blind GCP (taken from Pix4D website)

Getting 3D GCPs from Google Maps Sandbox on Daft Logic:

1. Open Google Maps Sandbox on Daft Logic: http://www.daftlogic.com/sandbox-google-maps-find-altitude.htm.
2. Search for the location of the project.
3. Zoom in up to the closest view possible of the area of the GCP.
4. Move the mouse over the point where the GCP has to be marked and click on the map. The 3D coordinates will appear in the box below the map.

https://support.pix4d.com/hc/en-us/articles/202560149-How-to-obtain-the-georeference-using-2D-or-3D-GCPs-taken-from-a-Web-Map-Service-#gsc.tab=0

SAV

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Re: Google Earth for GCP no geotag photos
« Reply #7 on: May 03, 2017, 08:49:04 AM »
Great link, Jeremia_ROWE.
Thanks for sharing.

Regards
SAV

Viking69

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Re: Google Earth for GCP no geotag photos
« Reply #8 on: May 03, 2017, 10:09:55 PM »
I found this document:

http://www.agisoft.com/pdf/PS_1.1%20-%20Tutorial%20-%20PS_to_GE.pdf

This seems to be a workflow that I am looking for but still have a question on step 3. the altitude field is set at "12" for each of the input google earth coordinates. How many z values do I need to have to orient the model so when I export it the model is reference "up". Can I just enter one z elevation as long as they have a positive value? Will this field create distorted vertical scaling if each x, y coordinate does not have a unique z value?

Thanks

Alexey Pasumansky

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Re: Google Earth for GCP no geotag photos
« Reply #9 on: May 04, 2017, 01:24:02 AM »
Hello Viking69,

You need to input the coordinates for at least three points that do not lay on the same line. Each of these points should have all three coordinates input (note that "zero" value means that the corresponding coordinate is zero and not that it is just absent/unknown).
Best regards,
Alexey Pasumansky,
Agisoft LLC