Documentation for 3-meter Orthophoto Tile

This data set contains a digitally rectified aerial photograph for the 12-km square region centered at latitude 40.9931°W, longitude 79.2124°N. The image has been resampled to 3-meter resolution in an Albers equal-area projection optimized for Pennsylvania. The source imagery for Pennsylvania was obtained by the USGS between 1992 and 1995; a new program of aerial photography was started in 1999/2000, but the results have not been incorporated into this data set.

The 3-meter digital image was created by merging a number of 1-meter-resolution digitally rectified aerial photographs, each covering slightly more than one quarter of a standard 7.5-minute USGS map quadrangle, into a 12-km square tile. The 1-meter "Digital Orthophoto Quarter Quadrangles" (DOQQs) were obtained from the USGS Eros Data Center (EDC). To build tiles in a single Albers projection for the entire state of Pennsylvania, the 1-meter DOQQ images from EDC were reprojected from the UTM projection to Albers before merging them together.

Adjacent DOQQ images overlap by several hundred meters. Since different images were obtained at different times of day and under different lighting conditions, there are sometimes abrupt changes in the average brightness when going from one DOQQ to an adjacent DOQQ. To reduce the abruptness of these changes, the merging of adjacent DOQQs used a weighted average of the intensities of the overlapping DOQQs within the overlap region; the weights used for a given DOQQ increased linearly from zero at the outside edges to unity once the non-overlapped region of the DOQQ was reached.

To facilitate rapid joining of adjacent images, the area covered by the available aerial photography was divided into tiles, each measuring 12 km by 12 km. The image for a region of interest which extends across two or more tiles can be assembled by extracting the required portion of each tile and then abutting these regions to each other.

To reduce the volume of data to be stored and processed for display, the original 1-meter resolution of the USGS DOQQs has been reduced to 3 meters by using only every third row and column of the merged 1-meter images. This resolution was chosen because it eliminates most of the blurriness of the 1-meter images while still permitting recognition of small roads and buildings.

The image data are presented as a single-band "band-sequential" (BSQ) array of 1-byte (8-bit) values containing 4000 lines (rows) of 4000 samples (columns) each. The data start in the northwest (upper-left) corner of the image, going west to east (left to right) along each line.

Projection and Geolocation Information

Projection parameters:
Name: Albers Equal Area Conic (centered on Pennsylvania)
Spheroid: GRS80 (NAD83 datum)
First standard parallel: 42° north
Second standard parallel: 40° north
Central meridian: 78° west
Latitude of projection origin: 39° north
False easting: 0
False easting: 0
Bounding coordinates:
Projection --
-108000 to -96000 easting
228000 to 216000 northing
Geographic (approximate) --
79.2848° to 79.1402° west
41.0478° to 40.9383° north latitude

Image resolution: 3 meters.

Image dimensions: 4000 lines (rows) of 4000 samples (columns) each.

Last change: 2002 Oct 9
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