Data sources and formats

This workflow for preprocessed orthophotos is designed for imagery with the following characteristics:

• Imagery has been orthorectified so that no georeferencing or geometric adjustments will be required.
• The source is typically an aerial camera system, but the imagery may have come from a satellite sensor.
• Bit depth is typically 8 bits per pixel per color but may be 16 bits.
• Imagery typically has been color corrected from one image or tile to the next, but in some cases color correction may not have been applied.
• Imagery may be single band (panchromatic), three color (true-color RGB or false-color infrared), or four band (RGB plus near-infrared).
• Imagery may be stored in many files, sometimes referred to as tiles, or alternatively, as large-area mosaics in compressed files.

If your data is not described above—for example, perhaps you have unprocessed imagery from a satellite or aerial camera—refer to other workflows in this guidebook.

You may need to consider alternatives in the workflow for each of the characteristics above (for example, 3 versus 4 spectral bands, 8 versus 16 bits). These options are discussed below, organized according to these three typical cases (in addition to cases A, B, and C discussed above).

Data case 1: Tiled orthophotos

These are the most common. They are typically created by the data provider from a collection of images that are individually orthorectified and then processed into a seamless mosaic, after which processed image tiles are exported from the mosaic.

• Images are stored as many regular tiles, in most cases filled with valid imagery (for example, no regions of NoData are present in the image files, except possibly in a small number of tiles around the project boundary).
• Tiles may overlap but are often edgematched.
• Tiles may have 4 bands but most commonly have 3.
• Dynamic range may be 16 bits but is most commonly 8 bits.
• Color correction will typically already have been applied.

Below are a number of separate tiles—a contiguous project area on the left, and imagery following a linear corridor on the right:

Image on left is courtesy of USDA APFO (NAIP, http://www.apfo.usda.gov/). Image on right is courtesy of Montana State Library (http://geoinfo.montanastatelibrary.org/).

Data case 2: Multi-image mosaics in large files

These image products are produced in a manner similar to data case 1, with the exception that the data provider will deliver a large contiguous mosaic image instead of individual tiles.

• The valid imagery area will typically have irregular boundaries, surrounded by large areas of NoData.
• Data is typically compressed, resulting in alteration of the NoData values (for example, NoData is typically zero, but after compression, some zero values are changed to values near zero).
• Color correction will typically already have been applied.

An example is shown below, representing two county compressed mosaics (CCM) from the United States Department of Agriculture (USDA) National Agricultural Imagery Program (NAIP). On the left, the black NoData areas obscure valid imagery; on the right, the NoData values have been properly hidden.

Data case 3: Individual orthorectified image frames

This case is the least commonly encountered. In this case, images are individually processed and then output as orthorectified files. They will typically not be cropped (and thus show NoData around each image) and may not have been color corrected. (If yes to both cropping and color correction, this becomes data case 1).