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Author Topic: Group or Split Calibration for Alignment of Historic Aerial Photos?  (Read 7901 times)

StevenF

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Hi All,
I'm trying to figure out a good workflow for aligning and generating orthomosaics of aerial photographs taken in the 1930's and 1940's. The photos were scanned at 14 microns using a photogrammetric scanner, but some potential problems with these photos include:
1. A lack of true fiducials - They have 4 corner marks but they don't look like true fiducials to me.
2. Poor film condition - some noise, scratches, variations in illumination and possible distortions.
3. Varying orientation - the corner marks don't line up at the same pixel locations in each photo and so the location of principal point is likely very different in each photo.

I've seen that a few other people on this forum have experience working with historic aerial images so I'm hoping to get advice on a few points given the issues with these images:
1. Can these photos be treated as from a metric camera and calibrated as a group or would I be better off splitting the groups so each image is calibrated separately?

2. If I decide to split the images, would I need to provide a better initial estimate of the principal point for each image? Like using the intersection of lines connecting opposite corner marks.

3. Would a decent estimate of the principal point and focal length be sufficient for good alignment or should I do additional image pre-processing?

4. During "Optimize Alignment" (after ground control) should skew and k4 be fit? I know skew is commonly 0 with images from modern digital cameras and k4 is probably negligible, but I'm not sure if the parameters would be appropriate for the images I'm working with.

I've attached a reduced resolution image of one scan to give a sense of what I'm working with. My goal is to generate orthomosaics of these photos that match a more recent orthomosaic (2011) with less than 5m RMSE. Any advice is appreciated.

Thanks,
Steve

photogrammetrix

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Re: Group or Split Calibration for Alignment of Historic Aerial Photos?
« Reply #1 on: September 08, 2014, 06:56:08 PM »
Hi Steve,

do you have any information about  the camera the images where originally  taken with? Are these military  aerial recon images? You may find something useful here:

http://www.airrecce.co.uk/

I was recently mosaicking some scans of German Luftwaffe recon images from WW2, and was lucky that most of the necssary information in the fotoframe was still present. Only one fiducial was cut of. Image and scan quality was quite good. Main problem with that kind of images in most case is lack of sufficiant overlap.They normally have only standard stereo overlap.

What do you have in mind with the fotographs? I think mosaicking may be possiible, but DSM generation probably not.

Cheers


StevenF

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Re: Group or Split Calibration for Alignment of Historic Aerial Photos?
« Reply #2 on: September 08, 2014, 08:32:22 PM »
No, unfortunately I don't have any info about the cameras, but I've been trying to dig some up. The images were taken by the Soil Conservation Service, and I'm guessing they used a Fairchild Camera with a 8-1/4" focal length lens. You can see in the posted image that used "Eastman Topographic Safety" film.

The goal is to look at changes in land cover. I'm going to try generating a DSM but I don't have high hopes for anything of good quality, and I don't have any field data from that time period to verify against. A good orthomosaic is all I really need.

I'm now thinking of doing a run with splitting the cameras and no pre-processing. If check points show poor accuracy then I'll go back and try adding some steps that might improve the results.

photogrammetrix

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Re: Group or Split Calibration for Alignment of Historic Aerial Photos?
« Reply #3 on: September 08, 2014, 09:09:12 PM »
Hi Steven,

I was wondering about the image format, because it is not a quadratic. Commonly most Aerial Mapping cameras come with a quadratic image format. A camera which has not, is the Fairchild A-18, it has 9 by 18 inch. If the zig-zag mark at the bottom is reallly a remain of a fiducial -  the shape really suggests this - it is astonishing how large the mark is, compared to the size of the whole foto. Is this really the complete original photo or was it scanned in parts and this is only such a part?

I think too, that spending too much effort into the calibration will not necessarily lead to better results, that is what I found while working with the above mentioned images too.

Good luck!
Cheers

bigben

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Re: Group or Split Calibration for Alignment of Historic Aerial Photos?
« Reply #4 on: September 09, 2014, 12:27:26 AM »
I'd at least crop or mask out the writing on the film. These can cause problems because they will  provide matches between unrelated images

StevenF

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Re: Group or Split Calibration for Alignment of Historic Aerial Photos?
« Reply #5 on: September 10, 2014, 08:35:25 PM »
I think the images are 9"x7". I've seen reference to this film size before but I can't find out what cameras used it. If I can get good results then I guess it doesn't matter what the camera is, but I would like to know just out of curiosity.

The obvious looking mark in the upper left corner is one of the corner marks. The other corners have similar marks but they aren't very visible. I'm going to try using the intersection of lines connecting corners to get an initial estimate of the principal point because it's probably closer than the image center.

My plan is to test whether preprocessing and splitting/grouping cameras really matters. I'll compare the RMSE of checkpoints in the resulting orthomosaics for each method using a small set of 4 images. I'll post up the results when I'm done.

photogrammetrix

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Re: Group or Split Calibration for Alignment of Historic Aerial Photos?
« Reply #6 on: September 11, 2014, 11:23:51 AM »
Hi Steven,

you mentioned the corner markings in the photos.

Commonly in an aerial mapping or surveying camera the fiducials and all the additional information we  normally find on the black image frame -such as altimeter, image counter, focallength information, time stamp, vertical gauge, etc  are exposed onto the film while the image is taken. So all these things are "fixed" parts within the camerabody.

Having this in mind I was wondering about the corner-markers in your images, which have no constructive connection to the image frame. My conclusion is, that these corner markers may be exposed to the film by some other means, but I think these are no markings like the fiducials from the camera frame as in  metric mapping cameras. I assume hat these markers therefore will not be usable for any kind of calibration.

Just some thoughts about it ...

Cheers


StevenF

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Re: Group or Split Calibration for Alignment of Historic Aerial Photos?
« Reply #7 on: September 23, 2014, 06:34:41 AM »
So I finally got around to doing my calibration and pre-processing test. I've attached the results with a short write up.

In short, the different calibration settings and the pre-processing didn't influence the orthomosaic accuracy by that much. This suggests that aligning fiducials and estimating initial camera calibration may be unnecessary for accurate alignment of historic aerial photos. However, I suspect that part of the reason for the high accuracy is that these scans are actually pretty good with the fiducials at fairly similar locations (~20 pixels diff) in all the photos.

photogrammetrix

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Re: Group or Split Calibration for Alignment of Historic Aerial Photos?
« Reply #8 on: September 24, 2014, 01:38:51 AM »
Hi Steven,

many thanks for sharing your experiences and publishing your results. Great inspiration and guideline for handling such things.

Thanks!

Cheers

Jack_in_CO

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Re: Group or Split Calibration for Alignment of Historic Aerial Photos?
« Reply #9 on: September 26, 2014, 10:09:38 PM »
Steven,

Agreed on the thanks! I appreciate the nice analysis and write up. This is an issue that I have been wrestling with and have had some success using a similar uncalibrated image approach. My attempts relied on ground control from modern images; in rural areas many structures exist in both the historical and modern aerial photographs.  Your use of "low lying rock features", assuming you are using large boulders which on a slope may suffer from ground creep might increase your errors. Bedrock outcrops would certainly help eliminate that as an error source.

Thank you again!