Understanding Oblique Aerial Imagery
GEOVISTA® technology features the ability to perform “direct image measurement” from oblique imagery through a rapid, low cost, patented method. Imagery is captured with detailed knowledge of camera positioning information through the use of a precision IMU (Inertial Motion Unit), GPS system and advanced software.
Traditional aerial imagery is collected with a single camera looking straight down, usually with 60% image overlap. This is referred to as aerial vertical photography. When this imagery is viewed in an analytical stereoplotter or softcopy system, a Photogrammetric technician can determine the 3D locations of objects seen in the stereo imagery. Orthophotography and Digital Terrain Models (DTM) are typical products. Orthophotography is user friendly because it can be loaded into a GIS/Mapping system where the location and distances can be easily determined as long as the measurements are made at ground level. But because traditional orthophotography provides only a single top-down view, it cannot be used to see or measure the sides of buildings clearly visible in oblique pictures.
Measurements Using DTM
GEOVISTA® technology is used to create traditional 3D terrain models and Orthophotography products. By viewing the derived DTM from the same perspective as the oblique imagery, every pixel at ground level in the oblique image represents a point in the 3D model. As long as the point selected is at ground level and lens distortion is taken into consideration, every image pixel represents a 3D location in the DTM.
However, being completely dependent on the use of an existing DTM for performing measurements can be a problem. A disadvantage of only using this approach is that accuracy of the location cannot be any better than the unknown accuracy of the available DTM. Further, the user must understand that measurements are based on the use of a DTM; meaning you can’t simply point to at building window to obtain a 3D location because you will actually be pointing at a ground location on the other side of the building. How can you measure the height of a building if the roof comes to a peak and does not have a DTM reference point directly below it?
Some of our competitors actually reverse engineer the camera location and orientation by registering their oblique imagery to existing orthophotography and digital terrain model. Because our imagery is automatically georeferenced by the camera system, we can provide more perspectives of every location at the same cost.
GEOVISTA® Direct Image Photogrammetric Measurement eliminates the need for a DTM or time consuming reverse engineering each image from unreliable data sources. You can measure anywhere in GEOVISTA® imagery without a DTM.