Abstract
Traditional photogrammetric mapping technologies are being challenged by high resolution Interferometric Synthetic Aperture Radar (IFSAR) systems. Madsen et al., 1993 demonstrated the first rigorous assessment of IFSAR for topography mapping using the NASA/JPL TOPSAR IFSAR system. Ten years later, the IFSAR technology has advanced to the point where airborne systems are capable of accuracies in the centimeters. The flexibility of IFSAR system deployment (day or night operation), near weather-independent data collection, cloud penetrating capability, and quick turn-around time is providing an alternative to the conventional photogrammetric technology.

IFSAR for topographic mapping uses two apertures separated by a "baseline" to image the surface. The phase difference between the apertures for each image point, along with the range and knowledge of the baseline, is used to infer the precise topographic height of the terrain being image. Intermap’s IFSAR system, called STAR-3i is a 3cm wavelength, X-band interferometer operating on Learjet commercial aircraft. Typical data acquisitions are for areas of 10 km across-track (range direction) and 50-200 km along track (azimuth direction), collected at a coverage rate of up to 100km 2 every minute. The output of high precision IFSAR datasets is accomplished by on-board laser-based inertia measurement data navigational and differential global positioning system (DGPS) processing to determine the precise position of the Learjet.

The STAR-3i system was recently modified to increase DEM relative performance, achieving up to 50 cm vertical RMSE, and to increase the orthorectified radar image pixel resolution to 1.25 meter from 2.5 meter. The IFSAR system generates an orthorectified radar image and a digital elevation model. The elevation model represents the elevation values from the first surface that the radar comes in contact with, for example tree tops and building tops. This elevation data is referred to as digital surface model (DSM). Additional software is used to automatically generate “bare earth” digital elevation models (DEMs) from the DSM. The bare earth elevation values are required to generate terrain contours for the topographic map.

The improved resolution of the image data has increased the accuracy to which topographic features are extracted. STAR-3i's sensitivity to surface roughness, soil moisture, and topography along with its viewing geometry (is analogous to low sun-angle photo) make it an excellent medium to successfully map vegetation (dense and medium forest cover, scrub, mangroves, plantation, orchards) and topography (land cover, cultural features, drainage, contours). The operational approach for using IFSAR data to generate M. Lorraine Tighe Submitted to Map Asia 2003, the 2nd Annual Asian Conference and Exhibition in the field of GIS, GPS, Aerial Photography and Remote Sensing March 30, 2003 topographic maps is presented. The IFSAR data is used within a softcopy photogrammetric workstation using traditional photogrammetric methodologies. The stereomate functionality and pseudo-stereo capability permits trained photogrammetric operators to extract topographic features from the IFSAR data.

Intermap Technologies has been successfully generating 1:20,000-1:50,000 Topographic Maps from the STAR-3i IFSAR data for the past six years. The main objective of this paper, which is on-going, is to investigate the performance of the recently modified Intermap IFSAR system for topographic mapping at larger scales. Examples are given of the pre-upgrade data and the new data showing the improvements that were achieved as STAR-3i matured as a commercial system. This paper provides statistical data to demonstrate the variability of features collected from the IFSAR data when compared to aerial photography. The outcome of this study is significant to the mapping environment that is dominated by traditional aerial photography methods. Results indicate that the higher resolution IFSAR data offered by the STAR-3i system, allow for more precise feature extraction resulting in the ability to map at scales as large as 1:10 000. Given that the IFSAR data processor is semi-automated to output both DEMs and ortho-images in near real-time with little operator interaction, its use results in significant savings in time and labor over traditional photographic mapping techniques. Topographic mapping from IFSAR data is here to stay and may revolutionize the mapping is conducted in the future.