Forum : Photosynth Lounge

Photogrammetric, Oblique Image Stitching, Pets Dressed in Clothes Photos… this is a place to chat and share stories with your fellow Photosynthers. Not all topics have to be about photography, this is a place to relax and chat about whatever you fancy.


Topic: Geo-Alignment: Roof or Base?

Report Abuse Report Abuse
Nathanael.Lawrence (Over 1 year ago)
Hello, all.

I've been wondering what the Photosynth team's advise is relating to how we align our pointclouds to the map.

Often times the satellite (or high elevation aerial) imagery provided is unfortunately not looking quite straight down on a building, but rather at an angle where you can see at least two of the walls as well.

In these cases, I've wondered whether I should align the pointcloud to the base of the building or to the edges of the roof. If I do align to the base of the walls I can see, this means that if the pointcloud is accurate, it will actually fall inside the roof on the opposite side of the building.

This is even more true in cases where the roof of a building overhangs the walls. If both the roof and the base of the walls appear in the pointcloud, which should we try to align the pointcloud to on the satellite imagery? My instinct tells me that you'll say the roof, but I'd like to read your words for myself.
michaeldenis (Over 1 year ago)
It's always better to fit the square hole onto the round peg.

More helpfully though, I expect that the aerial photographs are connected to the Atlas-like road maps using something as a reference.  The most likely reference are the roads themselves, leaving me to recommend the bases of buildings as the most useful setpoint for geo-aligning.  A tall building will overhang onto a road, making a synth geo-aligned to the roof be out of place.  On the other hand, the geo-aligned synth to a base on a tall building will look slightly off when one "Dives In" through Bing Maps.

My sense is that it's probably ok to leave the matter where most other things Photosynth-related now rest: in the hands of the artist.
TonyErnst (Over 1 year ago)
The perfectionist in me wants to say "always" use the base, but I think Michael is correct - you should do whichever looks right for the transition.
Nathanael.Lawrence (Over 1 year ago)
Hmm. That is interesting to me.

It was not apparent to me that the base of the buildings as seen in the satellite photos have any more accurate coordinates than the rooftops. Michael's point is well taken in the case of skyscrapers but I was thinking more of buildings with very few stories. 

Logic suggests that if the roads are accurately aligned then the bases of the buildings must be at their actual coordinates as well, but if the photo is not purely orthogonal to the current view (or at least very close to pure), then already the correctness of the coordinates of the entire scene is called into question, the further things are from the road. Yes, as the elevation of landmarks increases, the inaccuracy would be seen to multiply, but even at actual ground level things will have already begun to go wrong, since the satellite or plane was not pointing straight down, but rather slightly sideways, compressing distances between objects as you traverse the ground.
Nathanael.Lawrence (Over 1 year ago)
This essentially means that the next time that imagery is refreshed underneath our synths, there is a high probability that our alignment will show to be inaccurate unless the new ortho imagery is equally inaccurately placed as the first.

I can understand the objections to using the roof corners as guides (especially in the case of overhangs, unless you have specifically shot to get a pointcloud for the roof corners + edges), but the problem is simply that with correctly orthographic shots, no base will be visible and in shots that are slightly oblique, only half of the base (at best) will be visible, whereas the entire roof is very nearly always available to align to, barring surrounding trees, etc. There is also the case where the entire rooftop is sunlit, whereas what base is visible is hidden in shadow.
Nathanael.Lawrence (Over 1 year ago)
Before geo-alignment, I used to always notice that the geotag pushpin was placed differently in Bing Maps Ajax and Bing Maps 3D (with both versions using the same imagery and presumably coordinates). I always placed my pins to be centred correctly in Bing Maps 3D, as that is where I anticipated Photosynth to go (the Silverlight Map Control not yet having been publicly announced at that point).

More recently I have occasionally observed a similar disconnect when testing geo-alignment. I position the pointcloud on the map, but then when the alignment is tested, the pointcloud actually jumps about half a centimeter on my screen off of where I just saved the alignment before performing the swoop down to the first photo. Has anyone else observed this? I have to stress that it is an unusual occurrence, but I have seen it several times now.

In any case, I value the discussion.
GaryMortimer (Over 1 year ago)
I've also really started thinking about geo aligning properly, my snag is that when I create aerial synths using just vertical images the point cloud is normally not in the right plane, I can understand that. It would be nice to be able to rotate it 90.

The obvious thing is to try and get some obliques in as well, but I can't get enough love happening to make the connection.

Heres one I just tried in fact.

http://photosynth.net/view.aspx?cid=ada7f7b1-763c-4b57-97db-b60b293ba8a8
matroberts78 (Over 1 year ago)
When I geotag my syths, does that make them available via search engines (e.g. someone can bing my synth?)
Nathanael (Over 1 year ago)
matroberts78, geotagging your synth will embed your synth inside of Bing Maps, so anyone who is browsing that area of the map with the Silverlight version of Bing Maps ( http://www.bing.com/maps/explore/ ) should see your synth's thumbnail in the left hand pane.

Certainly, if they have launched the Photosynth Map App (use the right-most icon at the bottom of the left pane to open the App Gallery and choose Photosynth), then they should see your geotagged synths when they browse across the appropriate part of the map.

As far as just typing the title or tags of your synth into Bing web search or Google web search, if you include the word 'photosynth' in your query and the title of your synth is unique enough, it should turn up without needing to geotag it.

Feel free to clarify your question if I didn't quite answer what you wanted to know.
You need to be Signed In to add a comment. (Are you new? Sign Up for a free account.)