Seeking clarification re: Georeferencing/Orthorectification workflows

[JasperRomero]

Hi all,

I am hoping some folks with more experience and expertise can help clarify the best path forward for me:

I have multiple sets of scanned historic aerial images (1920s - 1950s) that I need to georeference. For the set I am working with now, there are ~200 scanned images I want to make into a seamless mosaic image after georeferencing. The scanned images include the whitespace from the pages around the edges of the images.

My workflow so far has been to load a tile, georeference with ~6 control points, Clip Raster to remove the whitespace border, and repeat. I have noticed that the edge alignment of resulting images is not always perfect - there are disjointed/offset edges where distinctive features don't align. The sidelap and overlap of the tiles is not fantastic - maybe 12-25%, varying by side and image - so I understand that the best possible outcome may not be perfect. I recently tried out mosaicking a few tiles together hoping that the "Blend" operation would help smooth the edges, but it did not.

I would like to ensure that I can produce the most seamless mosaic possible, so I have started to wonder if my workflow should be different - should I (or can I) be trying to orthorectify these images rather than just georeferencing? Should I clip the borders, mosaic/reference all the images to each other first to get more seamless edges, and then georeference the larger mosaic? What process and tools would you use to make the best possible seamless, georeferenced mosaic image down the line? Am I completely overlooking any steps/tools?

Any suggestions or insights are greatly appreciated! Thanks in advance!

[RichardDaniels]

The offset you mention "there are disjointed/offset edges where distinctive features don't align" should be expected. If these photos were taken with a airborne camera then you should be able to find the actual camera Lense that was used for the mission.  Pre 2000 all reputable companies had their cameras calibrated and certified by the USGS. Each camera/Lense combination have unique adjustments that would be added to your camera model to correct for distortion. 

When mosaicking cut lines should be created and only the most 'accurate' portion of each image retained in the final Ortho mosaic.  If you are using every-other photo within a mission or flight you will have approximately 33% overlap (and sidelap) between adjacent photos (you only need EVERY photo if your trying to create a DEM from the orthophotos).

If you are looking for survey grade accuracy for your resulting orthomosaic (sub meter) you need to look into the camera specs and research how they can be used to optimize your mosaic in ArcGIS Pro. The ArcMap and ArcGIS Pro Image Analyst extension has this capability.

[JasperRomero]

Thanks for your reply @RichardDaniels! I will see if I can find any more info about the camera and lens used for the flight. 

Given that the overlap and sidelap between any given images seems to be well below 30% in all cases, is it fair to say that I should not expect perfect alignment along the seams in the final product? My understanding is that the distortions for each image increase with distance from the nadir of each shot, but if I trimmed more than 5-10% around the edges of each image there would be gaps in the data.