Query By Elevation Versus Query By Depth

Defining a point in the earth

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. The basic ucvm_query gives user the option of query by depth (this is the default: -c gd) and query by elevation (-c ge).

Query by depth

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

Query by elevation

Based on the UCVM digital elevation model, we have determined this point has an elevation of xxx meters above sea level. To query the same point by elevation, we must determine the elevation of the query point. To determine the elevation of this point, we need to subtract the elevation from the depth value. Positive elevations are above sea level. Negative elevations are below sea level.

• (Elev at Lat/Lon) - (Query Depth below earth surface) = query elevation

We can determine the elevation of a point within UCVM's defined region by running a query at any depth. For example, if we query the above point by depth, we can obtain the elevation for the point from column 5 in the returned parameters, in this case 468.4m elevation.

[ucvm@c9392335cfb4 target]$ ucvm_query -f /app/ucvm/conf/ucvm.conf -m cvmh -c gd -l 34.0,-118.0,1000.0
Using Geo Depth coordinates as z mode.
 -118.0000    34.0000   1000.000    281.668    468.400       cvmh   2634.464   1098.676   2133.005       none      0.000      0.000      0.000      crust   2634.464   1098.676   2133.00

Then we input this elevation into our formula to determine the query value used in query by elevation:

• 486.4 - 1000.0 = 193.5m

Then we define our ucvm_query, and add the -c ge command line parameter to indicate this is a query by elevation. This defines the same point in the earth in an alternative form, as elevation above or below sea level.

• ucvm_query -f /usr/local/opt/ucvm/conf/ucvm.conf -m cvmh -c ge -l 33.84007,-117.95683,193.5

Query by Depth:

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:

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". Also query by elevation in the ocean, above the ocean floor, will typically return Vs=0 indicating the point is in the water.

UCVM Digital Elevation Model:

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.