April - June 2006
Staff members of Ayres Associates Inc. combined mobile GIS and digital photomapping technology to more quickly and accurately verify the location of alluvial fans on a proposed building site near Phoenix, Arizona.
Before any development could be planned on the 300-square-mile area west of the White Tank Mountains, the Maricopa County Flood Control District required an area drainage master study that would identify the location of water runoff features and ensure that construction could proceed safely.
Geologic field mapping was a critical component of the study. Ayres is an engineering firm based in Eau Claire, Wisconsin, and an Esri business partner. The company received a subcontract to perform a geomorphic assessment. The firm would evaluate potential development hazards by finding and delineating alluvial fans in the area. [Alluvial fans are cone-shaped deposits of sediment formed where a stream meets a flat plain.] To maximize the efficiency of its on-site mapping and minimize the need for follow-up fieldwork, the firm combined mobile GIS and digital photographic mapping technology to embed GPS-stamped field photos into the layers of its deliverable GIS products.
Mapping with GIS
Phoenix's urban sprawl is gradually spreading west along Interstate 10 to the small agricultural town of Buckeye where future development has been proposed. Concerned that the four known alluvial fans that might pose problems for the building of roads and neighborhoods, the Maricopa County Flood Control District requested a detailed geomorphic analysis of drainage features and soils in Sun Valley, a 184-square-mile area north of Buckeye.
When siting new construction, active alluvial fans should be avoided because the flow of sediment and rainwater from the nearby mountains within these formations is difficult to predict. As water travels through stream channels from higher elevations, its flow loses confinement when it hits the top, or hydrographic apex, of the alluvial fan at the base of the mountain. From that point, the water can flow with the force of a flash flood in any or all directions across the fan, wiping out any structures in its path.
Using ArcGIS as requested by the district, Ayres began creating a GIS of the project area at its Fort Collins, Colorado, office. The layers included Natural Resources Conservation Service soils mapping data; an Arizona surface geology map; a 10-foot topographic map; and aerial photography acquired from several different years, most recently 2003. These datasets were georeferenced and overlaid in the GIS.
Initial delineation of the alluvial fans was a two-step process. First, a visual analysis of the aerial photos identified most of the fans by their delta shape and lack of vegetation. Second, soils maps were overlaid so geologists could relate soil types to landforms to more accurately distinguish the components and edges of the fans. The soils data also assisted in differentiating the stability of the sediment in the various parts of each alluvial feature.
Geomorphic analysis in the GIS identified 16 new alluvial fans ranging in size from 10 to nearly 900 acres. With the fans delineated as a georeferenced data layer, the team prepared to head into the field to verify and refine these assessments. Field work would fine-tune the map layers generated in the office by mapping the perimeter boundaries of the fans and pinpointing locations of other key drainage features to an accuracy of one meter.
Mobile Mapping in the Field
A two-person field team traveled to the project site in Arizona during the first quarter of 2004. In preparing for on-site mapping, the team purchased a Trimble GeoXT handheld mobile GIS unit, an integrated GPS receiver and data collection device. After loading ArcPad onto the GeoXT, the team customized ArcPad with a pull-down menu containing various drainage features and attributes likely to be encountered in the field. The mobile GIS unit was mounted on one of two all-terrain vehicles (ATVs) the company rented for traveling through the rough terrain at the foot of the White Tank Mountains.
The team members uploaded the entire digital GIS dataset to the laptop computer that would remain in the hotel during each day's fieldwork. In addition to ArcGIS, GPS Photo-Link was loaded on the laptop. GPS Photo-Link, a software package from GeoSpatial Experts of Thornton, Colorado, would allow the team to integrate digital field photos into GIS layers with the coordinate location of each photo accurately embedded in the dataset.
Before heading into the field each morning, the team uploaded those sections of the 2003 aerial photo and alluvial fan delineation layers of the area that would be visited that day onto the mobile GIS unit. With the demarcated air photo on the GeoXT's screen as a guide, the team selected an alluvial fan and drove to it on the ATVs.
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