A digital orthoimage is a remotely sensed image data in which displacement of features in the image caused by terrain relief and sensor orientation have been mathematically removed. An orthoimagery combines the image characteristics of a photograph with the geometric qualities of a map.

High Resolution Digital Ortho Imagery Dataset (Year 2002):

For this dataset, the U. S. Geological Survey produced natural color orthoimages at 0.3-meter pixel resolution (approximately 1-foot). The design accuracy is estimated not to exceed 3-meter diagonal RMSE (2.12m RMSE in X or Y). Each orthoimage provides imagery for a 1500- by 1500-meter block on the ground. The projected coordinate system is UTM with a NAD83 datum. There is no image overlap been adjacent files. The naming convention is based on the U. S. National Grid (USNG), taking the coordinates of the SW corner of the orthoimage.
True color stable-base aerial photography film flown in year 2002 at 7,500 feet above average ground elevation, provides the imagery for the digital orthoimages. The USGS format elevation data with a 10m grid post interval were used to provide ground elevations for ortho-rectification process.

Rectification Process:

Horizontal and vertical controls were used to establish positions and elevations for reference and correlation purposes and as input to the aerotriangulation process. The ground controls consisted of Airborne GPS to provide camera station positions, photoidentifiable surveyed ground control for ground reference, and the final aerotriangulation solution for the project.

Each image was scanned from film-stable source at a resolution of 21 micrometers. Airborne GPS provided the camera station X, Y, and Z coordinates and was used to control a softcopy aerotriangulation solution. Available USGS photo-identifiable ground control points were used to test the results of the aerotriangulation adjustment. External orientation parameters derived from the aerotriangulation solution were used to provide the horizontal control for each photograph. Vertical control for the ortho-rectification process was obtained from various sources including existing USGS 10m DEM's, and mass point and break line data obtained by the City of Lincoln (Public Works and Utilities Dept.) to support 2- and 4-foot contour mapping. Multiple datasets may be merged to produce complete coverage for each 3.75-minute DEM generated for use in production. During the ortho-rectification process, elevation data were extracted and resampled to meet user-specified intervals and bounds.

Quality Control:

The project was worked in blocks approximately 1500-meters in extent. Every image was orthorectified except in the Lincoln downtown area where select images were eliminated as redundant. During photographic reproduction of the source photographs, limited analog dodging (radiometric editing) was performed to improve image quality.