- SPECIAL REPORTS
- THE MAGAZINE
HJW Geospatial Maps "The Rock.”HJW GeoSpatial (HJW), Inc. of Oakland, Calif., is recognized as a pioneer in the geospatial industry because local governments, federal agencies, utilities and private engineering firms have come to HJW for accurate mapping for more than 50 years. With several offices in California and Arizona, HJW’s custom professional services include aerial photography, digital orthophotography, planimetric and topographic mapping, remote sensing and GIS. Founded in 1949 as a forestry company, HJW continues to provide complete forestry services throughout the western states. HJW also operates Pacific Aerial Surveys, our photo library that makes more than 500,000 historic and current images available to the public. Despite the breadth of our individual services, however, the core of HJW’s practice is photogrammetry.
Photogrammetric mapping is a fairly straightforward process involving aerial photography, mapping and orthorectification. More often than not, photogrammetry is applied to properties such as landfills, condominium complexes and shopping centers. While the utility of these projects is undeniable, the glamour factor is decidedly low. That’s why the chance to work on projects with intrinsic geographical interest, such as HJW’s recent mapping of the infamous Alcatraz is particularly gratifying.
Architectural Resources Group (ARG), an historic preservation and planning architectural firm based in San Francisco, Calif., contracted HJW to map Alcatraz to support stabilization and restoration projects they are conducting for the National Park Service. Although a number of structures were burned or vandalized during the Native American occupation from 1969 to 1971, many still remain. Once the most powerful military fort west of the Mississippi River, Alcatraz still boasts gun battery emplacements dating back to the 1860s. Cellblocks that confined a total of 1,545 men during its tenure as a top-security federal prison are visited each year by thousands of tourists straining to feel the evil presences of Al Capone and Machine Gun Kelly.
The Rock’s ComplexitiesThe technical approach to mapping Alcatraz was not difficult to design, but the compilation proved to be something of a challenge. One characteristic of the island made project management easier, however; there was no confusion as to the limits of mapping.
The specifications for mapping were 1" = 40', with contours on a two foot interval. As it turns out, the island itself is shaped somewhat like a stereo model; it is about 1500' x 700' and fits comfortably in the stereo overlap of photography exposed at 1:3000. Our surveyor set four of the five control points at our design locations, but the fifth point had to be moved slightly because it fell in a sensitive nesting area on the northwest side of the island. Alcatraz was originally barren of vegetation, soil and water. Dirt brought in from Angel Island during construction of the fort accidentally introduced native grasses, annuals and trees, which continue to attract a large and varied bird population.
Our mapping photography was exposed at 1:3000. The compilation was interesting in two respects. The island has several buildings on it, the ruins of several others, and broad areas of cracked and decaying pavement. Interpreting and defining the edges of the crumbling infrastructure elements was not always easy. The other tricky component of the mapping was representing the topography. The island has more than 100' of relief, some curving engineered terraces, several irregular cliff faces and a tree-covered hillside. There is nothing terribly out of the ordinary here, except that all of these features are packed onto one small island.
HJW is currently in the process of generating an accurate and attractive orthophoto, which is also proving to be complicated. The abrupt changes in elevation and the structures built on steep terrain create smear and occlusions that have to be handled by special techniques. One exposure taken at 1:3000 contained too much relief displacement to be used practicallly. HJW plans to take a new exposure at another scale to see if that will be easier to orthorectify. If that fails, several new exposures will be collected around the island and mosaicked together to make an acceptable ortho image.
Though a few technical challenges arose while mapping “The Rock,” and although it’s captivating to work on such an illustrious piece of property, the fact remains that the technical practices employed bore little difference from landfill photogrammetric projects. Boring? On the contrary. The core of mapping’s fascination is that it provides accurate geospatial data for a wide range of land areas and structures. HJW has mapped the Golden Gate Bridge, the elaborate Hearst Castle at San Simeon and countless charming inns and vineyards in the Napa Valley. HJW even brought mapping into the art world when it applied close range photogrammetry for a Greek sculpture restoration project. Each of these projects posed unique scenarios, just as does each landfill and shopping center project.