This datalayer had previously been a mix of data from the DTMs as well as some 1:25,000 Digital Line Graph (DLG) data obtained from the U.S. Geological Society. As of August 2001 all older USGS data have been replaced by the 1:5,000 product. In April 2003 data development was completed for the entire state. The ArcSDE layer is named CONTOURS5K_ARC.
Arc/Info TINCONTOUR was used to create the DTM-based contours. Breaklines coded "hard" and "soft" were used in combination with "generate" files containing the "mass" and "spot" DTM points. All of these data were extracted via awk scripts on a Unix workstation from the original DTM files collected on stereo analytic plotters. The Orthophoto Index Grid coverage was used as a CLIP coverage as the DTM points overlap 5-10 percent in all directions. MassGIS staff performed extensive quality control and applied slight generalization and splining alogrithms in Arc/Info to enhance the cartographic display quality of the linework. These 3-meter contours are accurate to +/- 1.5 meters.
The layer's arc attribute table has the following fields:
|ELEV_M||4||5||B||Elevation above or below sea level, in meters|
|ELEV_FT||4||5||B||Elevation above or below sea level, in feet|
|INDEX_ARC||1||1||C||Coded 'Y' for all multiples of 15, to facilitate display of index arcs for labelling. All other arcs coded 'N'. This item is included in the SDE version and in shapefiles distributed by MassGIS. To select these index arcs in the data available for download use the query: MOD("ELEV_M",15) = 0|