IDRISMOD is translation software to go between IDRISI Geographical Information System (GIS) files and the U.S.G.S. Three-Dimensional Flow Model, MODFLOW (McDonald and Harbaugh, 1988). IDRISMOD translates between regular IDRISI images and MODFLOW array-images using MODFLOW variable grid spacing coordinates. IDRISI uses a grid of regularly spaced raster cells, usually of very small size. In contrast, MODFLOW typically uses grids of variable size with many fewer columns and rows. A MODFLOW grid defines nodes which may be either square or rectangular and usually cover larger areas than an individual IDRISI cell. IDRISI displays geographically referenced images in "real world" coordinates, while MODFLOW calculations involve distances between the centers of nodes, not geographic locations. These differences between IDRISI and MODFLOW are reflected in different file structures. IDRISMOD translates between the two types of files by using the same column and row vectors that are defined in the DELC and DELR arrays for the MODFLOW model grid. This translation software makes it possible to move mapped information developed for a site investigation and contained in the IDRISI GIS as layers, directly into MODFLOW input arrays without additional processing.

IDRISMOD first performs a forward translation where IDRISI cell values are spatially resampled into MODFLOW node values. During the forward translation there are several methods for sampling such as: arithmetic average, sum of values, maximum or minimum value or predominant value. After forward translation, a MODFLOW array may be displayed in IDRISI if geographic positions are defined through backwards translation. The translated MODFLOW array can be automatically pulled into a MODFLOW input data set, using GISMOD which is a preprocessor which can prepare MODFLOW input parameters and put IDRISMOD arrays in the right input format. There is also a utility which can be used to covert MODFLOW binary output files to MODFLOW array images. The model grid file for MODFLOW can also be viewed in IDRISI using another utility to develop vector files based upon the DELC and DELR arrays defined for MODFLOW. These utilities make it possible to create MODFLOW input data sets and view them in IDRISI.

The IDRISI GIS software provides options to read and prepare files from other commonly used mapping and GIS formats including: USGS DLG (Digital Line Graph) files, USGS CTG (Composite Theme Grid) data files, Map Analysis Package format files (both PC and Mainframe), pMAP image files, ODYSSEY vector files, ERDAS ".lan" and ".gis" format files, ARCINFO files, spread-sheet generated raster files, raster file conversion to vector or polygon files, generic band-interleaved images, and generic header files to IDRISI format. This capability makes it possible to move maps and GIS layers into IDRISI from most of the commonly used formats and then convert them into MODFLOW format using IDRISMOD.

The IGWMC/CSM-prepared, DOS-based programs support interactive text-based data entry and correction. The distribution disks contain runtime versions of the programs, an IDRISI data set, and a complete MODFLOW data set. IDRISI and MODFLOW are not included.

SYSTEM REQUIREMENTS

Intel 80386 processor with math coprocessor, 4 Mb RAM, hard disk with at least 3 Mb free disk space, DOS 3.0 or higher, graphics modes as supported by IDRISI, IDRISI GIS software program version 3.0 or later, U.S.G.S. MODFLOW model (IGWMC version 1.31 or later).

Authors: W.K. Conrad, A.K. Turner and S.S. Paschke (Colorado School of Mines/IGWMC).