Both coverages were created from an ARC/INFO hydrography coverage called MTHYGAP (created/edited from USGS 100k DLGs). MTRIVERP was created by selecting out double-line features, as well as islands, and building polygon topology. Attributes then were added to code for presence of islands within major river areas. MTRIVERA was built by manually selecting arcs from MTHYGAP to supplement the double-line features of MTRIVERP, which underrepresented the state's major waterways. This process was necessarily somewhat subjective in nature because stream order or other objective criteria for selecting features were not available in the source digital data.
MTRIVERP and MTRIVERA were never used directly in predicting vertebrate species distributions; rather, they were primary inputs used to create a number of grids that were actually used in the raster-based modeling process (see final Process_Step).
The following metadata elements are required by GAP, but do not parse using the FGDC ms parser (although similar elements can be found later in this document for several of these). For the convenience of GAP users, these elements are listed here. Data Set Identity: MTRIVERP, MTRIVERA; Raster File Format: NA; Raster File Sensor: NA; Vector File Format: ARCE7; Nonspatial File Format: NA; Source Distance Resolution: Unknown; Raster File Number of Bytes per Pixel: NA; Native Data Structure: Vector.
STEP 1. Identify the desired features: major rivers (those in MTRIVERP), large rivers (those in MTRIVERA), both, major rivers containing islands (polygons in MTRIVERP where TYPE = 888 and ISLAND = 1). If needed, create a vector layer of the desired features using the RESELECT command in ARC to extract the desired set from MTRIVERP into its own coverage. To produce some intermediate grids, the BUFFER command then was used to identify distances to the selected features. For example, "buffer river_arc rivarc1km # # 1000 # line # full" would buffer large rivers by 1000 m.
STEP 2. Enter the GRID module. Setmask and setwindow to ensure that the output grids are "snapped" to the state boundary grid with pixel increment, rows, and columns matching all other vertebrate modeling inputs. Specifically, the commands "setmask stategrid; setwindow stategrid mtgapveg" were used.
STEP 3. Use a CON(POLYGRID()) or CON(LINEGRID()) statement, then a CON(ISNULL) statement, to create the desired output grid (which, for modeling purposes, must contain values of 1 and 0, without any NODATA). For example, the commands "majrivtemp = con((polygrid(riverisland, type, #, #, 90)) == 888, 1, 0)" and "majrivgrid = con(isnull(majrivtemp), 0, majrivtemp)" would create a 90 m grid of major rivers where all cells within the state are coded 1 or 0. If a grid of both major and large rivers were desired, the user would next need to create a corresponding 90 m grid of large rivers: "lgrivtemp = con((linegrid(river_arc, type, #, #, 90)) == 777, 1, 0)" and "lgrivgrid = con(isnull(lgrivtemp), 0, lgrivtemp)". The two grids could then be merged into one: "rivertemp = merge(con(majrivgrid == 1, 1), con(lgrivgrid == 1, 1))" and "rivers = con(isnull(rivertemp),0, rivertemp)". (GRID users will notice that some of these steps could have been skipped, but because we wanted major rivers and large rivers in separate grids as well as a combined one, it was easiest to always use the CON(ISNULL) statement, although it made subsequent MERGE functions more complicated.)
Numerous grids were created following this sequence.