GISDEM

Synopsis

inputs a USGS Digital Elevation Model data file, and produces a regular Mesh usable by other AVS/Express modules

Input Ports

demFile	string
subsample	float
GlobalRadius	double
read_data	int

Output Ports

QuadName	string
Information	string
Status	string
demGrid	Mesh_Struct+Node_Data

Description

GISDEM inputs a USGS quadrangle Digital Elevation Model description file. USGS DEM files support four models: 7.5-minute, 15-minute, 30-minute and 1-degree. This module supports only the 15-minute, 30-minute and 1-degree models.

Note: UTM based quadrangles are not currently supported in GISDEM.

Because the USGS DEM quadrangles are of such high resolution, the data sets themselves are quite large. A typical DEM file is 10 Megabytes in size. Placing all of this information into an AVS/Express Mesh data field can result in an in-core data chunk substantially larger than the original 10 Megabyte file. To get around this potential problem, GISDEM provides its own subsample routine to decimate the dataset as it is being read in. By using this subsample feature, a user can read in a subsampled version of the DEM file on which to test the rest of his/her Express network. Once satisfied, subsampling can be turned off, and the entire file can be read in and passed through to the rest of the network.

The format for the USGS DEM quadrangle files was provided by the Digital Elevation Models Data Users Guide 5, available from the United States Department of the Interior's U.S. Geological Survey.

Input Ports

demFile

The fully qualified path name to the location of a USGS DEM quadrangle file.

subsample

A floating point value between 0.0 and 1.0 representing the amount of subsampling required on the DEM file. The value represents a percentage of the file that will survive the subsampling. 0.25 indicates that 75% of the file will be decimated, allowing 25% of the file through to the mesh, while 1.0 indicates the entire file will be passed through. (Recommended values are in the range of 0.05 to 0.1 for first time users.) The subsampling scheme is simple decimation of the data set: 0.25 indicates that only 25% of the data values from the USGS DEM quadrangle will be used.

GlobalRadius

This is a value that will be added to the altitude component of the DEM values. Otherwise, altitude values are typically represented as meters above or below the vertical datum.

read_data

The trigger port for the ingester. GISDEM will not read the data out of the file hierarchy until this port is set to a non-zero value.

Output Ports

QuadName

A 40 character, blank-padded string containing the name of the USGS quadrangle represented in the DEM file. This information is derived from the file's header record.

Information

A 40 character, blank-padded string containing additional information placed into the DEM file header. This is an optional field filled by the USGS that may contain useful information regarding the quadrangle, but it is often blank.

status

An information string that returns an english description of the results of the DEM quadrangle ingestion. A value of "File is ok," indicates that the quadrangle was successfully read, decoded and subsampled.

demGrid

An AVS/Express Mesh_Struct+Node_Data data field containing the information obtained from the quadrangle. The result is a square grid consisting of latitude, longitude and altitude values for the quadrangle.This Mesh data field can be passed through to GISMeshProject for map transforms on the DEM quadrangle.

About USGS Digital Elevation Model quadrangles

There is a lot of information contained in a USGS DEM quadrangle file concerning the type of data, model used, resolution, and so on Although the ingestion engine behind GISDEM reads and understands all of these fields, it was decided not to include ports for all these fields in this release of GISDEM. Most of them are not used, and the ones that are tend to have the same values for 15-minute, 30-minute and 1-degree data files.

USGS DEM Quadrangles are available from a variety of sources, such as the Internet (search the World Wide Web for "USGS DEM"), and the USGS publications office.

For a complete understanding of what is in the USGS DEM quadrangle files, please refer to the US Government publication Digital Elevation Models Data Users Guide 5, available from the United States Department of the Interior's U.S. Geological Survey.

Example

Not Available.

File

v/gis3.v