VLM

Description

Parameters

Parameter descriptions

FILE

Specifies the name of the PCIDSK image file to undergo volume analysis.

DBIC

Specifies the input channel that contains the elevation data.

DBIB

Specifies the bitmap segment (Type 101) that defines the area for which the volume will be calculated.

ELEV

Specifies the base elevation for volume measurements. Volume is calculated between the surface of the elevation image and this elevation level.

PXSZ

Specifies, in meters, the X and Y dimensions of one pixel on the ground.

If this parameter is not specified, the function uses the pixel size stored in the input file. Correct calculation of the volume depends on proper specification of this value.

ELSZ

Specifies, in meters, the elevation corresponding to one gray level.

A change of one gray level in the elevation image corresponds to a change in elevation of ELSZ (Elevation step) meters. Correct calculation of the real and projected areas depends on proper specification of this value.

IMSTAT

REPORT

Specifies where to direct the generated report.

Available options are:

• TERM: generates a report on the terminal (default)
• DISK: generates a report on file "IMPRPT.LST"
• OFF: turns off report generation
• <filename>: appends a report to the specified file

MONITOR

The program progress can be monitored by printing the percentage of processing completed. A system parameter, MONITOR, controls this activity.

Available options are:

• ON: Turns the monitor ON (default)
• OFF: Turns the monitor OFF (recommended if running in batch or background mode)

Details

VLM uses an elevation channel stored on the database file to calculate the volume between the surface of a bitmap-defined region and a user-specified base elevation.

The volume between the surface defined by the elevation data and the specified elevation base is calculated under the bitmap mask. Note that volume can either be positive (elevation is above base elevation) or negative (elevation is below base elevation).

The total absolute volume (sum of absolute values of positive and negative volumes), in cubic kilometers, is written to the first IMSTAT array element (Sum of Abs. Vol.). The total net volume (difference between positive and negative volumes), in cubic kilometers, is written to the second IMSTAT array element (Sum of Net Vol.). The positive volume is written to the third IMSTAT array element (Sum of + Vol.), and the absolute value of the negative volume is written to the fourth IMSTAT array element (Sum of - Vol.). The calculation results are also written to the report device.

Correct calculation of the volume is dependent on the proper specification of the elevation step size and the ground pixel size. If defaulted, the elevation step size is set equal to 1.

If defaulted, the ground pixel size is read from the database header.

Example

Calculate the volumes defined by the bitmap for the irvine.pix data set. Each pixel in the image represents a projected ground area of 30 meters by 30 meters, and each increment in the gray level of the elevation image corresponds to the default elevation change of 1 meter.

Recall that the 16-bit elevation data is stored on channel 10, and that the base elevation is 112m.

EASI>file	=	"irvine.pix"
EASI>dbic	=	10
EASI>dbib	=	11	
EASI>elev	=	112
EASI>pxsz	=	
EASI>elsz	=	1
EASI>imstat	=
EASI>RUN VLM	
EASI>print "Total absolute volume: ", imstat[1]
EASI>print "Total net volume     : ", imstat[2]
EASI>print "Total positive volume: ", imstat[3]
EASI>print "Total negative volume: ", imstat[4]

 VLM     Elevation Data Volume
  
 irvine.pix                  [S   11PIC     512P     512L] 14-Aug-90
    10 [16S] MCD     USGS Elevation Data                    1-OCT-90

 Volumes in cubic kilometers under bitmap  33:

 Absolute volume =    1.813463    
 Net volume      =    1.790463    
 Positive volume =    1.801963    
 Negative volume =  -1.1500200E-02