Why does the impervious layer not exactly match up with the imagery layer?
The impervious layer does not always match up exactly with the imagery because of the angle that the aerial photography was taken at. The only accurate part of the photograph is the exact center of the image. All other features in the photo are radially distorted outward from the center. The more elevation a feature has, the more distorted it is from the center. In order to adjust the distortion the elevation has to be known throughout the photo. In order to fix this the imagery is geometrically corrected down to its original position. The ground terrain information is then modeled but not the above-ground features such as buildings. Therefore, the base of the building on the ground is adjusted to the proper position but not the top, causing it to remain distorted on the image. The only way this can be resolved is with true orthophotography, in which buildings are modeled in 3-D.

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1. How was the aerial imagery collected?
2. What is orthophotography?
3. How was the impervious data collected?
4. What if my home or building was built or significantly changed after 2009?
5. Why does the impervious layer not exactly match up with the imagery layer?