Here's an interesting video of a photogrammetry project processed with LPS using Kite Aerial Photography. The video starts starts with terrain contours and then drapes imagery on top. Impressive work when you consider it was created with a digital camera and a kite.
When discussing true orthophoto generation I made reference to the image mosaicking process. I thought I would touch on that more today, with an emphasis on high resolution imagery. One of the main challenges in mosaic production is ensuring the mosaic is seamless. That is, one cannot easily discern where the edges of the input images are. This can be challenging for a number of reasons. One of the most difficult aspects involves the input image geometry. Because the input images have different perspective centers, the geometry of surface objects will vary between images. For example, a tall building in the center of a frame image may not exhibit any building lean, but the same building in the next image will show noticeable lean. So the big challenge in the mosaic process is ensuring the seams between input images conceal any mismatches. While seams between the images can be automatically generated, a quality control check must be performed to ensure there are no issues. Seams usually need to be manually edited if there are any problems.
The recently-opened World Digital Library is another great resource for historical maps (see here for another, and also see the recent post at VerySpatial).
The main page allows you to filter results by date range and region and then you can narrow your search with a number of options, one of them being "maps". The maps can be viewed in flash, with zoom control, and it is also possible to download a TIF image.
The map below is a 1507 world map that was first to depict the Western hemisphere with the Pacific Ocean as a distinct ocean (click on the image below to see the details). Check out the site if you're interested - looks like they have over 300 maps already online. Details about the project are available here, and it an associated Slashdot discussion here.
The previous post covered an explanation of true orthos, and in this post I wanted to outline a few notions on true ortho creation. As discussed previously, true orthos can be very expensive. This is because of the extra effort that goes into production. However, there are a couple of different ways to produce true orthos. These include:
True Ortho Processing
Flight Planning and “Managed Mosaicking”
These two methods both have pro's and cons and are more applicable in some circumstances rather than others.
The first method refers to special techniques used to create true orthos. The ortho photography page at Eastern Topographics outlines the pre and post-processing results quite well. The technique that is typically applied involves the use of input images and a bare-earth terrain model (just like regular ortho processing), but with an additional component of 3D building features that need to be captured in stereo via a 3D feature extraction software. So the raw ingredients to the process would be (a) input imagery, (b) bare earth terrain, and (c) 3D feature data. It is the latter component that typically drives up the cost of production. Because the buildings are typically extracted manually, the human cost of collection gets bundled into the true ortho pricing. As for what actually goes on in the processing, a comment on the previous post provided an excellent link to explain the details of automated true ortho processing. It also outlines the importance of color matching, which is important for achieving acceptable results. The other thing to keep in mind is to ensure there is enough valid pixel data, otherwise occluded areas (aka the areas of the image obscured by building lean) in the input imagery may be filled with black void pixels. This can be alleviated by ensuring the imagery was collected with a high enough overlap percentage.
The other method that is often employed is to simply fly the project area with a very high degree of overlap (e.g. 80/80 forward/sidelap versus the usual 60/40). Then orthos are produced for all the frames via the usual approach with bare earth terrain. During the mosaick process, the operator can then interactively select the image portions (the center area of each frame) via seam editing, mosaick the images and then tile them back out into whatever their specification requirement calls for. This approach may not be applicable for high urban environments (e.g. Manhattan) but can work well for suburban and low-rise building with a few high-rises here and there. While fuel costs are going to be higher because of the increases overlap, the processing costs should remain low.
Note that pushbroom sensors such as the ADS80 can be ideal for the latter approach. This is because they can capture imagery at nadir in long strips, which dramatically reduces the number of input images into the mosaic process. Here's a screen capture of ADS80 imagery taken from the middle of the strip. While the multi-story buildings along the edges of the strip have discernible building lean, the imagery at the center doesn't show any. Flying with high sidelap would allow for the inclusion of just the central areas for the mosaic processing. It may not be perfect 100% of the time, but I would argue that it is good enough for many applications, without requiring the high cost of collecting all the building features.
Digital Orthophotos have become a premiere geospatial data products in recent years. Although they are often used as background context for the display of vector data, there is quite a bit of complexity that can go into creating them. If you've ever looked into purchasing orthos, you may have been given the option of buying "true orthos". This was the case with yours truly several years ago at my first job out of university, when I was tasked with purchasing orthophotos for several metro areas in the USA. Compared with "regular orthos", true orthophotos seemed outrageously expensive... So what are True Orthos?If orthophotos can be characterized as images that are geometrically corrected for relief variation, true orthophotos add the dimension of correcting for the distortion of buildings. Or, simply stated, true orthophotos do not show building lean. This is important for mapping applications such as digitizing street centerlines in Lower Manhattan. "Normal" orthophotos will show displacement of skyscrapers and many of the streets will be obscured. It isn't a major issue in suburban or rural environments, but may be necessary for urban environments such as Hong Kong, NYC, Seoul, and other metro environments with a large number of skyscrapers. True orthos can also be important for transportation planning projects, such as accurately mapping bridges.Here is an example: the ortho of central Los Angeles below shows building lean that is common in many urban environments. The facades of the skyscrapers are clearly visible, and the surrounding areas on the ground are obscured. Yahoo Maps for the same area shows a similar effect.
This next image shows what a "true ortho" would look like (a different part of central LA). The only way to tell the buildings are highrises, other than the giveaway helicopter landing pads, is the long shadows cast by the two buildings in the center.
Here is another application featuring a couple of buildings in Lucerne, Switzerland. Although these buildings are not skyscrapers, the effect is clearly visible. The building facade is readily displayed in the image below. 
The next image, of the exact same building, shows a top down view that does not display building sides.
Nadir view without building leanNext Post: why true orthos are expensive and some notes on true ortho production.
