FAQs • Falls Church, VA • CivicEngage

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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GIS Mapping

1. How was the aerial imagery collected?: The City’s most current imagery is from 2009 and was collected as a part of the Virginia Base Mapping Program (VBMP) Orthophotography project (http://www.vita.virginia.gov/isp/default.aspx?id=8412). The Commonwealth of Virginia contracted with the Sanborn Map Company to provide 200 scale, one-foot pixel resolution, and natural color orthophotography to the Virginia Geographic Information Network (VGIN). Updates to imagery are dependent on the state’s acquisition schedule.

2. What is orthophotography?: Orthophotography is aerial photography that is geometrically corrected so that it has a uniform scale. Unlike an uncorrected aerial photograph, an orthophotograph can be used to measure true distances because it is an accurate representation of the Earth’s surface, having been adjusted for topographic relief, lens distortion, and camera tilt.

3. How was the impervious data collected?: Impervious features, including pavement, parking lots, driveways, and buildings were also mapped by Sanborn as part of the VBMP in 2009. The features were hand-digitized from the 2009 one-foot resolution orthophotos, and include structures with roofs and paved/concrete surfaces having an area coverage of 200 square feet or greater.

4. What if my home or building was built or significantly changed after 2009?: The City of Falls Church GIS Analyst consulted all post-2009 grading plans to determine the correct impervious coverage area. The GIS Analyst received the 2013 aerial imagery in April and will verify or correct the existing impervious surface data against the new imagery.

5. 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.