Data Processing

Raw Data. The DEM data was received on over 30 CDs in floating-point precision binary format (BIL) tiles, in UTM WGS 84 projection, at both 5 m and at 10 m posting. The ORRI data was received in GeoTIFF tiles, projected in UTM WGS 84 projection, at 2.5 m posting. The data arrived in over 120 tiles, which then had to be mosaicked together. Receiving the data at both resolutions and merging the tiles turned out to the most time consuming and onerous task of this project, due to several miscommuincations with Intermap at the outset of the project and the fact that the mosaicing process could not be truly automated in either ArcInfo or Imagine due to the header formats (which also changed during the course of this project). We have included a copy of the raw DEM data in this distribution. It is hoped that in future distributions that Intermap will be able to deliver their data either as complete files on DVD or offer a custom mosaicking tool.

Mosaicking. The raw DEM data includes ArcInfo compatible header files in addition to NSDI compliant header files. In order to speed up the process of data merging we used FLOATGRID in ArcInfo to import the BIL tiles to ArcGrid tiles and merged the grids using MERGE in ArcInfo. The DEM spans UTM zones 5 and 6. The result was a DEM mosaic for UTM zone 5 and a DEM mosaic for UTM zone 6, both in ArcGrid format.

Reprojection. We imported the ArcGrid files into Erdas Imagine in Imagine's native *.img format for further processing. The UTM zone 6 mosaic was reprojected to UTM zone 5 using Imagine's Raster->Geometric Correction feature; Reproject as geometric model, and a Polynomial Order setting of 3. Nearest neighbor was used as resampling method. After the UTM zone 6 data had been reprojected to UTM zone 5 it was merged with the existing UTM zone 5 data using Imagine's DataPrep->Mosaic Images feature. At this point the original NODATA value of -9999 was replaced with 0. The resulting mosaic of the entire Kuraruk watershed was exported to binary format using Imagine's Import/Export utility. The error introduced by reprojecting the Zone 6 data was estimated by Imagine to be less than 0.5 m over the width of the DEM.

Resampling. Because many potential users will likely not require the data at full spatial resolution, we resampled it to 25 m, 50 m, and 100 m postings. We did this using Imagine's resampling tool, using both nearest neighbor and cubic convolution methods, starting with the 10 m data set provided by Intermap. Our understanding is that Intermap uses custom software to produce the 5 and 10 m posting data directly from their SAR processor. We started with the 10 m data set because of a data dropout (see Data Validation for details) in one location that was only corrected with a 10 m patch by Intermap; that is, no 5 m patch exists.

ORRI Processing. The ORRI tiles have been merged using Imagine's DataPrep->Mosaic Images feature to create one mosaic for UTM zone 5 and one mosaic for UTM zone 6. The UTM zone 6 mosaic has been reprojected to UTM zone 5 using Imagine's Raster->Geometric Correction feature; Reproject as geometric model, and a Polynomial Order setting of 3. Nearest neighbor was used as resampling method. After the UTM zone 6 data had been reprojected to UTM zone 5 it was merged with the existing UTM zone 5 data using Imagine's DataPrep->Mosaic Images feature. We resampled the ORRI to 5 m and 25 m postings and exported these files to GeoTIFF format. It was not possible to to include the 2.5 m posting as GeoTIFF as the required filesize would exceed the 2 GB TIFF standard. The 2.5 m ORRI as well as the 5 m and the 25 m, are provided in Imagine's *.img format.