Scanning aerial photography into digital imagery can greatly assist GIS and CAD technicians by saving time and reducing potential errors. Digital imagery is used as a real world back drop for comparison and updates of vector or polygon data. The main purpose of scanning systems is to replace the task of automated tracing (digitizing), to assist in remote sensing applications (land use/land cover characterizations), as well as raster-based modeling. Visualization, through imagery, is an essential form of spatial analysis which allows for more informed decisions to be made. Imagery can greatly assist in a variety of project applications:
Air photos are available in a variety of scales, decades of coverage, and for most regions, states, and counties. Perhaps the one limitation to aerial photography is the actual spatial coverage of individual photos (9-25 sq. miles vs. 1400-13,500 sq. miles for satellite scenes). This limitation can be handled in several ways; 1) scanned photo images can be merged or mosaicked in image processing software, or 2) if greater spatial coverage or higher spectral resolution is required for a project, then satellite imagery could be considered. Air photos are, however, relatively inexpensive compared to satellite imagery, have high spatial resolution, and are archived by numerous state, federal, and private agencies. For this reason, aerial photography is particularly well suited for pattern comparisons between seasons or over periods of time.
The following is an example of a project which may find scanning air photos useful, time saving, and cost effective.
The USDA and SCS have expressed a need to quantify the extent to which agricultural land use has changed, both spatially and temporally, in the Southern Appalachian region. According to USDA statistics, the Southern Appalachian region has experienced a decline in farm acreage of over seventy percent from 1950-1987. The SCS predicts a parallel reduction in associated agricultural soil loss during the period. This project will require the analysis of multiple years of air photo coverage to determine the extent of land use change and to ultimately to perform soil erosion modeling. A GIS environment would be the best way to manage and store the data for this project. For the entire region, it is expected that photo scale and relative quality will vary through the available years of coverage. Once the remote sensing analyst has determined availability, the GIS technician will be responsible for scanning the aerial photography into an image format that the GIS can import. Most GIS software packages are able to import a variety of formats, however, this should always be determined prior to scanning by the technician. The digital images should be organized and managed by the GIS technician in order that other GIS project staff may access them.
Proper data management and archive organization is essential since only subsets of the entire region can be processed at a given time.
The following details the expected skills which students should master for each level of training (i.e. Awareness, Competency, Mastery).
Students should have an understanding of air photo scanning, potential applications of scanned images, spatial scale and file size considerations, and knowledge of image types and terminology.
The learning goals of this section are to develop the ability to scan aerial photography and other images, and to understand issues regarding spatial resolution and associated project application needs.
The learning goals of this section are for students to be able conceptually apply their understanding of photo scanning to any number of scanning software packages in a number of different lab environments. Scanning software include: Adobe Photoshop, Kodak Photo Edge, Corel Draw, Aldus Photostyler, Kodak Photo CD, and an increasing number of others.
Recommended:
Complementary:
Learning Objectives:
Vocabulary:
Topics:
1. Unit Concepts
Learning Objectives:
1. Students will be exposed to essential prerequisite information regarding the processes and considerations involved with scanning.
2. Students will be able to select and understand the implications of dpi selection as it relates to the output file size, and the desired spatial resolution requirements of the project.
3. Students will have a thorough understanding of basic image types.
4. The students should consider dots per inch and scale determination as well.
|
air photo scale: |
10,000 |
24,000 |
40,000 |
|||
|
kilometers across: |
2.29 |
5.5 |
9.1 |
scanned file size in pixels/Mbytes |
||
|
DPI Scan |
Pixel size in meters |
pixel width |
color vs. greyscale in megabytes |
|||
|
150 dpi |
1.7 |
4.1 |
6.8 |
1,350 |
5Mb |
2Mb |
|
300 dpi |
0.8 |
2.0 |
3.4 |
2,700 |
21Mb |
7Mb |
|
600 dpi |
0.4 |
1.0 |
1.7 |
5,400 |
83Mb |
28Mb |
|
1200 dpi |
0.2 |
0.5 |
0.8 |
10,800 |
334Mb |
111Mb |
5. The student should have an understanding of the basic graphic file formats:
NOTE: If the project intends to georectify the imagery or create orthoimages, additional considerations need to be made regarding radial distortions, and capture of the fiducial marks on the aerial photography (most scanners can only scan 8.5" wide which will not capture the fiducial marks). Fiducial marks are necessary for ortho-rectification, so larger scanners are needed. Because the principle point (focal point) of a photograph is the most accurate, capture of the far periphery of a photo (particularly in areas with complex terrain/vertical relief) can affect geo-rectification and rubber sheeting processes later during image processing.
Learning Objectives
1. Students will be able to use scanning operations to perform an air photo scan, and will consider the following list when at the scanner.
2. Exercises should be performed by students at various dpi scanning resolutions to observe visual differences in the resulting imagery, as well as to experience file size issues associated with different image types and greater spatial resolutions.
NOTE: Although this unit deals with scanning processes, before beginning a project ask if the data agency provides the coverage pre-scanned. Increasingly, more agencies are archiving analog air photos by scanning them and in some cases mosaicing certain series. Some agencies offer scanning refined services that can also prove to be cost effective.
Suggested: