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Volume No. 12, 2003/2004

Land Cover

Digital Aerial Photograph Interpretation: Examples and Techniques from Arizona and the Southwest Regional Gap Analysis Program

Keith Pohs and Kathryn Thomas

U. S. Geological Survey, Southwest Biological Science Center, Flagstaff, Arizona

Introduction

The Southwest Regional Gap Analysis Program (SWReGAP) is developing land cover maps using a biophysical modeling procedure that incorporates satellite imagery, maps of environmental variables, and extensive reference observations of vegetation types as model input data. Field crews have collected these reference observations throughout the five-state SWReGAP region―Arizona, Colorado, Nevada, New Mexico, and Utah. However, field investigation sometimes is not possible or is prohibitively costly. The former is due largely to limited access to certain lands. The latter occurs in large roadless areas where access is mainly by foot, often in extremely rugged terrain, and field points are less efficiently obtained. Since the biophysical modeling approach is most effective where there are sufficient observations for each vegetation type and adequate geographical representation across its occurrence, in Arizona we have used a method of digital aerial photograph interpretation to collect additional observation points. Using Digital Ortho Quarter Quadrangles (DOQQs) downloaded from the Arizona Regional Image Archive (ARIA, http://aria.arizona.edu/index.html) Web site or Digital Ortho Quadrangles (DOQs) interpreted on the TerraServer Web site (terraserver-usa.com), the Arizona team was able to delineate over a thousand additional vegetation observation points and polygons, classified to ecological systems as developed by NatureServe. In this paper we present the methods used to obtain the vegetation observations using these digital sources and the limitations and advantages of this methodology for regional mapping of land cover.

Methods

Two methods involving the use of DOQQs or DOQs were developed, depending on whether the imagery was accessed from the ARIA Web site or the TerraServer Web site. Initially DOQQs were downloaded from ARIA for various parts of Arizona. Images from 1992, 1996, and 1997 were selected by quad sheet name, based upon a GIS grid layer of quads for Arizona, and then systematically added as a layer into ArcMap. A land cover analyst was able to distinguish a number of ecological systems and delineate them as polygons. Initially we needed additional ground reference observations for a large area of the Sonoran Desert where field sampling was not possible due to access restrictions. Several ecological systems were discernible, including Sonoran-Mojave Creosote-White Bursage Desert Scrub and Sonoran Palo Verde-Mixed Cacti Desert Scrub.

Some ecological systems seemed underrepresented in our field observations. This was the case for a number of different reasons, but one was the placement of roads (our primary field sampling corridors) in areas of gentler topography relative to the surrounding landscape. This was apparent on the Colorado Plateau where travel routes simply avoid the steep slopes and bedrock expanses of the Colorado Plateau Mixed Bedrock Canyon and Tableland ecological system. These predominantly barren features, however, are readily discernible in aerial photography, and we obtained adequate representation of this type through interpretation of DOQQs. We also obtained reference observations for dunes, playas, cinder cones, and lava flows in this region and throughout the state. All of these features and their associated sparse vegetation are classified into a described NatureServe ecological system. Other ecological systems were digitized based primarily on vegetative cover and included Rocky Mountain Aspen Forest and Woodland and Rocky Mountain Gambel Oak-Mixed Montane Shrubland.

The ARIA site had considerable DOQQ coverage for much of Arizona except for the central part of the state. An alternative source of remotely sensed data was sought for these areas as well as areas in adjacent states for which the Arizona team has mapping responsibilities. This led to the development of a second methodology using imagery available on the TerraServer Web site.

DOQs for all of Arizona and adjacent study areas of Utah and New Mexico are available on the TerraServer Web site. The site hosts USGS aerial imagery from 1997 and scanned images of USGS topographic maps from various years. The team's land cover analyst obtained reference observations by navigating to a particular region or feature on the topographic maps and then switching to the aerial photograph for that site using the built-in features of TerraServer (Figure 1a).

 Invasive Southwest Riparian Woodland

Figure 1a. Interpretation of the Invasive Southwest Riparian Woodland and Shrubland Ecological System along the Little Colorado River, Arizona, using TerraServer. A USGS digital topographic map of this same view is a related link on the toolbar to the left of the image.

Alternatively, the analyst opportunistically scanned areas for discernable systems using the DOQs and then found the location of the type on the Web site’s digital topographic maps. TerraServer does not readily support systematic downloading of DOQ images and thus digitization in ArcMap nor does it provide sufficiently accurate geographic coordinates for locations of interest. We determined the geographic coordinates for identified ecological systems by determining the center of the system on the Web site’s digital topographic map and then finding the same location on a state National Geographic Digital USGS Topographic Map Ô (National Geographic Maps, San Francisco). Using the navigational TOPO! Software Ô and the compass tool that is part of this product, the UTM coordinates for the interpreted sites were obtained (Figure 1b).

Digital Topo Map of Riparian Woodland.

Figure 1b. National Geographic digital topographic map of same site in TOPO! view. Using the available compass tool, a UTM coordinate can be obtained.

We were able to develop several hundred ecological system points using this methodology and state National Geographic Digital Topographic Maps for Arizona, Utah, and New Mexico. In Arizona, data was obtained for a number of higher elevation areas that had not been sampled on the ground. This technique was also used in the roadless Gila and Aldo Leopold Wilderness areas in southwestern New Mexico. The most common ecological systems delineated in these regions included aspen forests and Gambel oak shrublands, as the dominant deciduous species were readily discernible from nearby coniferous systems.

The preponderance of private land along stream and river corridors, the occasionally steep topographic relief of canyon environments inhibiting foot and automobile travel, and in some areas wilderness designation can make the efficient collection of field points in riparian areas exceedingly difficult. The TerraServer remote sensing methodology again allows for some interpretation of vegetation communities in these otherwise inaccessible but ecologically important areas. Several NatureServe ecological systems are discernible, including Invasive Southwest Riparian Woodland and Shrubland (Figure 1a) and Rocky Mountain Lower Montane Riparian Woodland and Shrubland.

In some instances more points of a particular ecological system in a mapping area are needed for modeling than were obtained in the field. One working unit comprising much of northeastern Arizona and adjacent New Mexico, aspen forests, though locally common above 8,500 feet along the length of the Chuska Mountains did not appear to be sufficiently well represented by field observations to be mapped. Using the TerraServer methodology, additional points were obtained, and this important system may now be mapped.

Discussion

Use of the Web-based imagery allowed the Arizona SWReGAP team to acquire reference data that otherwise would simply not be available or available at a far greater cost than feasible for the project. It allowed the collection of data in areas where access was not possible on the ground. It also allowed us to increase the geographic representation of our reference observations, augmenting those collected in the field. The imagery used for the work is available free of charge on the Internet, and the only material cost was the purchase of commercial digital topographic maps on CD-ROM for two states, Arizona and New Mexico. The major limitation of both methodologies is that only some ecological systems are discernible given the resolution of the imagery used. Most of the higher elevation coniferous systems, such as spruce-fir and limber-bristlecone pine, cannot be easily interpreted. Ponderosa pine and pinyon-juniper systems can be interpreted but were not a priority for this work, as a considerable number of field observations had already been obtained. The technique does require familiarity with the ecological systems being mapped and their expression on the landscape. In our case, the land cover analyst doing the photointerpretation had spent extensive time on the ground acquiring reference data and was able to directly use this acquired knowledge in the interpretive work. In addition, the analyst identified known field reference data observations of ecological systems that were targeted for photointerpretation on the digital imagery to verify his interpretation.

The digitized polygons of this work can be overlaid on the developing map, or point locations can be extracted from within the polygons for inclusion in the training data, along with other photo-interpreted points. These and other training data are then used to produce the new GAP land cover map for Arizona and portions of adjacent states.

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