Introduction - Aerial Sketchmapping

The principle of aerial sketchmapping

Aerial sketchmapping for Forest Health Protection has been conducted since the 1940s. Typically, sketchmapping surveyors fly the area to be surveyed in a high-winged, high-performance aircraft at elevations of 1000 – 3000 feet above the ground. The surveyor tracks the plane’s location on a hardcopy map (typically several maps that are trimmed, edge-joined and taped together) or aerial photographs, then sketches areas of the forest that have been damaged during the past year (Figure on the right). The sketched features (points and polygons) are then registered and digitized into a Geographic Information System (GIS). Finally, the resulting GIS data are used to create maps and reports at forest, regional and national levels.

Although skilled surveyors have successfully employed manual sketchmapping techniques for years, there is great potential for error in the process. Some examples of the sources of error are:

• not knowing the aircraft’s location on the map
• the reinking of map data upon completion of the day’s flying
• inaccurate registration, line following, and attribute capture when digitizing data into the GIS.

In addition to the potential for error, some of the processes used in manual sketchmapping require additional time to complete. Examples of those processes that can be eliminated with a digital system are:

• the reinking of map data upon completion of the day’s flying
• the management of numerous taped and folded maps in the cockpit
• the taping and folding of maps prior to the mission, the cutting apart of those maps prior to digitizing, and the retaping of the maps after digitizing, but prior to proof-checking the digitizing results
• the digitizing process itself.

The positional accuracy of sketched features is largely determined by the ability of the sketchmapper to keep track of the aircraft’s position on the map and to correctly relate features seen on the ground to the map. This can be especially difficult in unfamiliar or flat terrain or both. At typical airspeeds of 100 -140 mph, a sketchmapper has approximately 15 seconds per square mile to accurately locate, sketch, and attribute features (assuming a visual mapping distance of 2 miles from the aircraft). Accuracy is a relative term, and the end use of the data must be considered. If the data collected during a sketchmap mission is for general monitoring purposes, then more error can be tolerated than if field crews need to locate the affected stands using the sketchmapped data.

To be favorably compared against manual sketchmapping, a competent digital aerial sketchmapping system should address the problems listed above and should meet the following list of requirements, which were compiled for early system development:

1. The map display must be linked to a GPS receiver so that an icon on the display represents the current position of the aircraft on the movable digital map.
2. The map display must update quickly at ground speeds of 130 mph and reorient when turning, if desired by the surveyor, based on the GPS position and heading of the aircraft.
3. The screen must be viewable under a variety of lighting conditions including full sunlight and must also display, at minimum, full (256) color.
4. The viewable screen size must be at least 10 inches measured diagonally.
5. The software must have the capability to:
• digitize points, lines, and polygons – including nested and overlapping polygons
• attribute digitized features (points, lines, and polygons) quickly and easily
• allow editing of feature attributes quickly and easily
• allow the user to predefine the feature attributes to be collected during that day’s mission
• collect the GPS log file of the flight
• allow the user to zoom quickly to different map scales
• retrieve and display common raster and vector data types as background maps quickly and seamlessly (including the USGS 1:100,000-scale DRGs aerial surveyors currently use in hardcopy form)
• export files in a format that is easily imported into ESRI Arc/Info software, thus eliminating the intermediate digitizing step
• save data automatically to the computer’s disk.
6. The hardware should be operational in moderately harsh conditions (32 – 120 ºF, high humidity, dusty conditions).
7. The primary input device must be a touchscreen with stylus.

After a great deal of testing and evaluation, the Forest Service successfully met these requirements, and two systems emerged and are in common use today; a two-screen system and a pen-tablet system. Advantages of the Digital Aerial Sketchmapping System (D-ASM) include automatic tracking of the aircraft’s position on a map base through a link to a GPS receiver and a significant reduction in the time spent moving data into a GIS. The D-ASM greatly speeds data collection and processing and improves the positional accuracy of the data being collected. Now a mature system, the D-ASM has been widely embraced by the sketchmapping community with over 90 systems in service with Federal and State forest health agencies.

Following initial system development, a D-ASM was shown to wildfire Air Attack crews who liked the concept, but wanted the system to be smaller and for it to include a means to get the information down to the incident command in real time. Two technological developments solved these requirements: commercially available touch-screen pen-tablet computers and the NASA-developed long-range high-speed data link named RIPCom.

The Forest Service is now working to further expand the utility of the D-ASM by adding live downlink capabilities to the existing system.