How is velocity calculated in Quikslope and Autoslope?
Velocity is the difference between adjusted distances of the current reading with the previous reading, expressed in mm per day. So if we have 2 readings: 1. 12-OCT-03 3:00AM adj dist=856.234 2. 12-OCT-03 1:00PM adj dist=856.236 3. 13-OCT-03 9:00AM adj dist=856.235 4. 13-OCT-03 9:00PM adj dist=856.232 The current reading is #4. Now the calculation in Quikslope (as at 25-May-04) will get the difference between #4 and #3 as 3mm over 12hours which equates to a velocity of 6mm per day. This method can be misleading because the instrumentation tolerance is +/- 3mm plus, if a weather station is not used, then atmospherics can increase that even more, although the REF prism adjustment should compensate. So, for very short time frames, say like 1 hour between readings, and if there is a difference of 2mm in distances, this will equate to 28mm per day velocity when indeed there may be a zero velocity. So to try to make the velocity calculation more practical, we do it a little different in Auto
