Query By Elevation Versus Query By Depth
There are alternative ways to define a specific point in the earth. Assuming the point is below ground level, the same point can be described by lat long and depth below the solid earth surface, or as lat long and elevation above or below sea level.
The basic ucvm_query command support either mode. The query by depth below earth surface is the default.
These two queries are equivalent. Both are query by depth, and will retrieve earth properties at 100m depth: ucvm_query -f /usr/local/opt/ucvm/conf/ucvm.conf -m cvmh -l 33.84007,-117.95683,100.0 ucvm_query -f /usr/local/opt/ucvm/conf/ucvm.conf -m cvmh -c gd -l 33.84007,-117.95683,1000.0
Based on the UCVM digital elevation model, we have determined this point has an elevation of xxx meters above sea level. To query the e ucvm_query -f /usr/local/opt/ucvm/conf/ucvm.conf -m cvmh -c ge -l 33.84007,-117.95683,0.0
To determine the elevation of this point, we need to subtract the elevation from the depth value:
- (Query Depth below earth surface) - (Earth Surface Elev) = query elevation
- (100m) - (xxxm) = xxx
The basic ucvm_query gives user the option of query by depth (this is the default: -c gd) and query by elevation (-c ge).
These queries differ slightly:
Query by depth, the user specifies a numeric value (in meters) at or below the surface. A depth of 0 is at the surface, a depth of 100 is interpreted at 100m below the surface. So depths are nearly always positive numbers.
Query by elevation, the user specifies a numerical value (in meters) of elevation above or below sea level. An elevation of 0 is sea level. an elevation of 250 is an elevation of 250 meters above sea level. An elevation of -100 is 100 meters below sea level.
Please note that the user can query for elevations above the surface of the earth, which is "in the air".
To support query by elevation, UCVM contains a digital elevation model for California. The elevation of a given point is returned in column 4 of the return parameters. User can query a model, using any elevation or depth, and UCVM return the elevation of that point in column 4. The user use this information to determine whether the elevation is at the point, and then can query at that elevation to get material properties at the surface for that point.
