YSI SonTek RiverSurveyor, System Manual

Page: 40 / 194

SonTek/YSI Inc
RiverSurveyor System Manual (May 1, 2007)
28
3.3.6. Bottom-Track Terminology
This section describes some of the key bottom-
track terms.
Track Length
The total distance traveled along the full length or
course of the vessel track (
Figure 3-10
).
Distance Made Good (DMG)
DMG is the length of the direct line between the
vessel’s start position and the vessel’s current po-
sition. For example, if you were to travel in a
complete circle, the track length would be the
length of the perimeter (circumference) of the cir-
cle, but the DMG would be zero because you
have, in effect, not moved from the starting point
(
Figure 3-10
).
GC-BC
GC (GPS course) is a direct (straight) line from
“start to finish” for the course traversed at the sur-
vey site using the GPS as the track reference. BC
(bottom-track course) is the same as GC, except
that bottom-track is used as the track reference in-
stead of GPS. GC-BC is the angle (in degrees) be-
tween these two lines (
Figure 3-11
). Ideally, this
value should be close to zero as both reference
systems are tracking the course of the same vessel.
If GC-BC is more than a few degrees, it is possi-
ble that the river has a moving bottom at that loca-
tion. If the value is consistently off by several de-
grees, it is also possible that the
magnetic declina-
tion has not been set correctly within the River-
Surveyor program.
D
BT
/D
GPS
This is the ratio between the DMG calculated using bottom-track and the DMG calculated using
GPS (
Figure 3-11
). Ideally, this value should be close to one. If the ratio is greater than one, it is
possible that the river has a moving bottom at that location.
Figure 3-10. Track Length vs. Distance Made Good
Figure 3-11. GC-BC and D
BT
/D
GPS
illustration. When
bottom-track and GPS-track are identical, the
D
BT
/D
GPS
ratio will be equal to 1.