A field map can be used for the Bfield by loading in an RZ field map description from a url.
<fields> <field type="RZFieldMap" name="RZFieldMapTest" gridSizeZ="10.0" gridSizeR="10.0" numBinsZ="64" numBinsR="67" url="http://www.lcsim.org/test/solenoid_5tesla.dat"/> </fields>
The format for the plain text data file is:
z r B z B r
The units are centimeters and kiloGauss.