Minggu, 01 Februari 2009

Oblique Imagery

From my perspective the market for oblique aerial photography has gone from a very niche application area and morphed into something altogether different. With the advent of organizations like Pictometry and applications of their photography such as the "Bird's Eye" view in Microsoft Virtual Earth, oblique imagery is entering a number of market spaces. Oblique aerial photography is not new though, however early attempts to commercialize the technology such as Kodak Citypix from several years back never really got off the ground.

I would be interested in thoughts on usage of oblique photography versus nadir "top-down" orthophotos. My view is that nadir photography is most appropriate for "measuring" objects within photos (GIS "backdrops", building areas, parcels, roads, etcetera), whereas oblique imagery is better for contextual information. So from a "mapping" perspective traditional top-down imagery may be more appropriate but for a navigation system my preference would be for oblique.

From an end-user/consumer perspective there is certainly a lot of value in oblique photography. One problem with nadir imagery is that it can be difficult to tell how tall buildings are, or gather any information about buildings (or anything other features with a "vertical" aspect). Oblique photography allows users to see the sides of buildings and other objects, which has a lot of appeal for a variety of applications.

The downside of oblique imagery is that, depending on the angle, objects in the foreground (e.g. highrises) can obstruct the view of anything behind them. They can also be difficult to integrate in a GIS, as most GIS apps feature a "top-down" perspective when you're looking at the data. For photogrammetric applications, it also isn't very practical to work with high oblique images in stereo.

Here's an exampe: the image below is a nadir view from Microsoft's Live Search Maps in Pacific Beach, San Diego. One can clearly see roads, all the buildings (from a top-down perspective), and the coast.


The image below is the same area with the "Bird's Eye" oblique perspective view. It immediately becomes apparent that there is a highrise along the coast, which obstructs the view and casts a shadow on several buildings behind it - useful to know if you're in the real-estate business. The bluffs along the ocean are also exposed.


And finally, here is another oblique aerial photo of the same area taken facing east. This is a closer-range and lower-altitude oblique shot than the images above, so a lot of building detail can be seen (e.g. one can determine the number of stories of the properties along the coast). This image also illustrates how the objects behind the highrise are obstructed - you'd need to take a look at the "top-down" nadir view to know what is there.


Click on the thumbnail above for higher resolution

Incidentally, the above photo is from the California Coastline Records Project. If you haven't heard of it, a visit to the site is highly recommended. The project has coastal oblique coverage of nearly the entire California coastline, taken from a helicopter approximately 500 feet offshore. The project has essentially created an online repository documenting the status (and damage to) the coastline. In celebrity news, the project was unsuccessfully sued by Barbara Streisand, who took issue with online photographs of her Malibu compound...
ARMU

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