How can I get the edges of images in an aerial mosaic to match better?

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04-22-2017 04:15 PM
JohnFraser
New Contributor III

I made a mosaic of aerial images of the island of Saipan from 10 images shot by the military in 1945. The mosaic worked but the images don't match up well at the edges. This is most obvious with roads and other straight lines. I used 30-50 reference points per image and a few different algorithms. I settled for a first degree algorithm because the individual images looked less distorted at the edges with that one.

1. How can I get the edges to match up ie, be continuous from one image to another? I tried dividing the images into smaller areas but the smaller areas don't provide enough referencing points to make a good mosaic.

2. How many reference points and which algorithm would be best for this effort: first degree, second degree or one of the others?

3. The images overlap closer in flat areas than on hilly and mountainous ones even though the images, I assume, were taken from directly above. Does that make sense in this context?

4. Unrelated to mosaics but how do you suggest I make the ocean the same value?

Mosaic of the island of Saipan from 10 aerial images taken in 1945.

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4 Replies
PeterBecker
Esri Regular Contributor

Check out the OrthoMapping capabilities in Pro 1.4.  They do required the advanced version of Pro. Essentially the only way to remove the distortions is to orthorectify the imagery. This requires the determination of camera parameters such as camera orientation based on knowledge of camera focal length and set of tie points and control points. The orthorectification process then corrects for camera and DTM values. The DTM you should be able to extract from WorldElevation services on ArcGIS Online. Alternatively if you have sufficient overlap then the DTM can be computed as part of the orthomapping workflows.

The other alternative is to measure a lot more control points and use Adjust transform. To use adjust you need to ensure the control points are well distributed. Look to use the auto-georeferencing tool, based on existing georeferenced imagery and then delete points that are wrong or tool close to each other. You will want to also add some dummy points in the Sea to keep the extents from warping too much.

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JohnFraser
New Contributor III

Thanks for the suggestions. I will have Pro 1.4 soon.  I am having trouble installing the licenses since I replaced the HD recently.

Regarding the dummy points at sea, how do you do that? Thanks again.

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CodyBenkelman
Esri Regular Contributor

John

Can you clarify exactly what you mean by "mosaic"?  I think I remember an earlier post on this topic, and I hope I convinced you to use a Mosaic Dataset, which is our data structure for managing and mosaicking imagery "on the fly" which means you can tweak and adjust the image alignment and see the results instantly.

If you're using a GP tool "Mosaic To New Raster", please put that effort on hold and we'll come back to the question of "what do you need as your final output?"  (e.g. will you use it in ArcGIS, or perhaps ArcGIS Online, or some other software?...)


Since you asked about 1st vs. 2nd order rectification, I assume you're manually entering tie points using our Georeferencing toolbar.  You've run into a classic problem of trying to use a mathematical model (in these 2 cases, polynomial equations) to fit imagery to terrain, and except in flat terrain, it is impossible with those choices.  The good news is there are choices that WILL work.  Try the ADJUST or SPLINE methods.  Note for spline you need at least 10 tie points per image, but I'm expecting you'll have at least that many.


Next, some other important advice:  

  1. Put your tie points on terrain extremes (peaks, ridges, AND valleys) as much as possible. 
  2. Use the imagery base map ONLY for tie points.  If you're inserting tie points to neighboring images, that creates other challenges.  (If you already have tie points to neighbor images, backup your work but then delete those tie points)
  3. do NOT use "Rectify" but instead use "Update Georeferencing".  The former choice writes a new image (single image for the input image) to disk which takes up disk space and time and creates data management challenges.  "Update Georeferencing" will write an imagename.aux.xml file to disk which will reference your original, unmodified image and project it onto the map.

Once you have a good set of tie points for each individual image, if you add those images (the original unmodified image names) into a mosaic dataset (MD), the MD will create a mosaic on the fly, and then you can work with seamlines if desired to improve the blended appearance of the composite image.

If you're using the results only in ArcGIS, the MD is all you need.  If you're publishing to ArcGIS Online, you'd want to Generate Tile Cache (and then copy that cache to ArcGIS online for publishing.  For this workflow I can point you to additional tools).  If you're sharing the final result with someone else using other software (e.g. Photoshop etc.) you would "Export" at the appropriate resolution.

If this isn't enough or if some of this isn't clear, let me know.  Will you share the results with us??

Cody B. 

JohnFraser
New Contributor III

Hi Cody, Thanks. Yes I posted about this a while ago. I stopped working on the mosaic when I found the edges of the images so poorly aligned and didn't know what to do about it. I'm a little more comfortable in arcmap now so I thought I'd try it again.

By mosaic, I mean putting the ten images together to form a larger image The images are all referenced to the same basemap which at the moment is a 2011 edition by the US Dept of Agriculture, I believe. I reference the images one at a time with 30-50 points, then display all 10 images for the whole map.

I don't recall using "Mosaic To New Raster" but I did explore a few tools when I first started this project.

I just now made a Mosaic Dataset with "Create Mosaic Dataset" and will see how that goes with my 10 referenced images and their tie points.

Next time I reference I will follow your suggestions about tie point placement. Yes, I use "Update Georeferencing", not "Rectify." 

I'll write more as I work on this. Thanks so much.

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