Each image covers a 7.5 minute quadrangle with a 300 meter overedge between quads. The DOQs are mosaicked to remove visible seam lines due to characteristics of the source aerial photography. Mosaic cutlines are interactively defined within the DOQ image overedge by examining each DOQ pairwise with adjoining DOQs and placing the seam line where image tone differences and geometric misalignments are minimized. Due to the overedge between ortho quads, and depending on the placement of seam lines, each ortho quad can contain portions of as many as 16 DOQs. During the mosaicking process, the image tone of all input DOQs is adjusted towards a common value.
The primary purpose of the ortho quads is to support USDA Field Service Centers located in counties throughout the United States. The DOQs are edge matched or seamed across county boundaries. In counties that are split by UTM zones, ortho quads are reprojected into the predominate zone. Ortho quads may be produced and distributed in an incomplete form when constituent DOQs are not available at production date.
Image Tone Adjustment
The area of coverage that images are matched within are individual counties. Image tone adjustment is performed for two primary reasons:
1. To produce a well contrasted image so that users are not required to adjust the tone upon display or to create an enhancement lookup table upon delivery.
2. To reduce visible tone differences between mosaic seam lines.
Methods of Image tone adjustment:
-- Manual Contrast Adjustment Each input DOQ image is viewed and, if necessary, the contrast and brightness is adjusted.
-- Automatic Contrast Adjustment Each input DOQ within a county is also run through an automatic tone or range adjustment that drives all input images toward a common value. The intention is to reduce vignetting and "hot spots" from the source photography, and to minimize tone differences along mosaic seam lines.
-- Seam Feathering Tone adjustment between input DOQs is adjusted along a gradation a specified distance out from the seam line. This smooths or feathers abrupt tone differences which would otherwise highlight seam lines.
Defining Mosaic Seam Lines
Input DOQs are displayed pairwise so the operator can decide where, within the overedge area, to place the seam lines. In general, the goal is to reduce visible seam lines due to differences in source photography.
Specifically, the goal is to: 1) Present single contiguous agricultural fields as though they are from a single date of source photography. 2) To minimize the visible effects of geometric misalignments between DOQs, such as an offset in linear field boundaries where adjoining DOQs meet.
The operator continually emaluates image tone differences and geometric misalignments between adjoining DOQs and interactively places the seam where both are minimized. DOQs are edge matched, or seamed, across county boundaries.
Mosaic Generation
Image 7.5 minute quad tiles are either generated individually and sequentially as work progresses through a project, or cut out from a larger mosaic after it is generated. Since the imagery is being resampled to the same coordinate system, nearest neighbor resampling is used.
Rules for Seam Line Placement:
1. Misalignments less than 3 meters with same photography dates.
- No Restrictions on seam line placement.
2. Misalignments less than 3 meters with different photography dates.
- Place seam in non-agricultural areas such as roadway easements, waterways etc..
- Place seam in same apparent landcover type or conditions in agricultural areas such as pastures, fallow fields or cropped fields.
3. Misalignments greater than 3 meters with same photography dates.
- If possible, avoid crossing linear features in agricultural areas with seam line.
4. Misalignments greater than 3 meters with different photography dates.
- If possible, avoid crossing linear features in agricultural areas with seam line.
- Place seam in non-agricultural areas such as roadway easements, waterways etc..
- Place seam in same apparent landcover type or conditions in agricultural areas such as pastures, fallow fields or cropped fields.
This file was generated by compressing mosaicked digital orthophoto quadrangles (MDOQs) that cover a county. MrSID compression, with mosaic option, was used. Default values for the compression ratio are (20:1) and compression levels(10) are used.