TrueOrtho imagery helps eliminate expensive stereo compilation for planimetric mapping.

ISTAR Americas and VARGIS LLC have teamed to deliver a one-foot, natural color TrueOrtho mosaic for the entire area of Howard County, Maryland. The fast-growing county is relying on the ISTAR TrueOrtho image mosaic to “heads-up digitize” new planimetric updates into their GIS as a cost-effective alternative to expensive stereo compilation. Howard County is a mixture of expanding suburbs and rural farmland along the Interstate 95 corridor between Baltimore and Washington, D.C.

Working as a VARGIS contractor, ISTAR Americas acquired the 1-foot resolution digital aerial data in late February 2002 and delivered a countywide TrueOrtho image to VARGIS within 90 days. This geometrically accurate mosaic was then color-enhanced in subsequent weeks to match past imagery, and integrated to the county’s GIS operation. Typically, the county obtains new stereo photography every four years to update its map base through traditional stereo compilation, but this year it chose ISTAR as a faster and more cost-effective way to update their planimetric landbase.

Howard County is one of several large-area mapping projects in which ISTAR Americas has used the HRSC-AX airborne camera developed by DLR, the German Space Agency. This digital camera acquires imagery at 25cm (~10in) pixel resolution from an altitude of 20,000 feet, allowing it to cover huge tracts of land in a single swath. In addition to 24-bit natural color and infrared imagery, the ISTAR cameras also produce 16-bit panchromatic imagery and perfectly correlated, high-resolution 3D surface models.

TrueOrtho image mosaics are ISTAR Americas’ signature data set, although the company also produces one-meter resolution 3D surface models. TrueOrtho data are derived from multiple swaths of digital HRSC-AX image data, and orthorectified using an automated pixel-by-pixel stereo production system. The result is a TrueOrtho image in which all surface features are visible – even alleys, streets and the sidewalks between tall buildings – and a realistic 3D model that’s an exact match to the image data.