Creating and Interpreting Digital Orthophotographs

Ground Validation

Following careful evaluation of the IKONOS imagery, and in some cases creation of a "first draft" habitat map through the process outlined in the previous section, selected sites were visited in the field for typological validation. This validation included:

1. areas in the imagery with confusing or difficult to interpret signatures
2. transects across many representative habitat types occurring in different depths and water conditions
3. a survey of the zones
4. confirmation of preliminary habitat delineations if a first draft was produced

Navigating to field sites was accomplished in a variety of ways including:

• uploading position coordinates from the mosaic into an onboard GPS and navigating to those waypoints
• using an onboard PC connected to GPS allowing navigation using digital nautical charts or the mosaic
• visual navigation using landmarks visible in the imagery

On most occasions, field activities were conducted with the guidance of local experts. IKONOS imagery, and when available, draft delineations were used in the field to facilitate comparison of signatures in the imagery to actual habitats at each site. Individual sites were visually evaluated by snorkeling and free diving or directly from the boat in shallow, clear water. Habitat transitions were evaluated by swimming transects across habitat types to further guide placement of polygon boundaries.

Habitat type(s), zone, approximate depth, position (GPS), image number, and other descriptive information were recorded at each site. Field data for each site was then compiled into a text table with a latitude/longitude field to allow overlay of the field information on the mosaic and habitat polygons (Ground Validation Points). Where depth and water clarity permitted, the IKONOS imagery were used to navigate across multiple bottom features allowing continuous confirmation of habitat types and transitions between each site.

Once the field data was processed, polygon boundaries and habitat classifications were created or revised where necessary, and zone attributes were assigned to each polygon using the Habitat Digitizer. This draft of the habitat maps was then reviewed and revised with the guidance of a panel of local experts at peer review sessions at several locations throughout the U.S. Pacific Territories, and over the Internet. Review session participants included members of the local research and management community.

During these peer review sessions, particular attention was given to polygons labeled as "unknown" and areas not visited during ground truth activities. Revisions based on comments from local experts were then completed and final habitat maps were produced. Thematic accuracy was assessed for these final maps (see Assessment of Classification Accuracy).