Gross
errors, also known as blunders. This type of error can be of any size or
nature. They generally occur due to carelessness of the person doing the
measuring, like writing down the wrong value or measuring between the wrong
points. This error can be reduced or eliminated by using good procedures but
transposing numbers is not uncommon.
Systematic
errors are errors that can be mathematically modeled and hence corrected. They
are caused by using a different mathematical model than that which exists in
the real world. A good example of a systematic error is using the slope
distance instead of the horizontal distance. This type of error can be reduced
by using the correct and complete model.
An incomplete model will add to your error budget and cause the final
measurement to be skewed.
Random
errors also have an effect on the measurement but have no apparent cause. This
type of error is the small differences between repeated measurements of the
same quantity. They can be reduce or eliminate them from our measurements by
statistical procedures. One example would be to create a mean value for a
measurement to be used for a later measurement.
Now I’m no
mathematician or statistical wizard, but I have been around the block a few
times in the error buggy. Feedback from others in our profession has always
been sharp to criticize a procedure but have also been faster to adopt a
procedure once it has been proven beyond a reasonable doubt. This reminds me
how some of the latest talks have pointed to the issue of “can scan data
produce results that are more accurate than the published specifications of the
scanner?”. Now for the answer… “YES” This is because there are many points that
are being used to solve the position of a point. In other words we can use the
power of averages to solve the final position.
The term
survey accuracy can be confusing, misused and politicized especially when a
layman is involved. Although the fields
of mapping and land surveying have benefited greatly from technology there is still
the resounding question of accuracy in cadastral surveying based on
measurements or law. There are accepted errors in the National Spatial
Reference Systems (NSRS), the Continuously Operating Reference Stations (CORS)
and the horizontal and vertical control stations maintained by the National
Geodetic Survey (NGS). There are errors in the equipment, human errors
introduced by the field surveyor, and accepted errors in the technology. But
never before have we dissected and expected such precise accuracy in our
profession until we knew we could obtain it. What was accepted as standard
practices with traditional surveying in the past is now not good enough with
current technology. The days of dead reckoning stood less criticism than .02’
laser scanner acquired data. Chaining has become obsolete which reminds me of a
story about the old man that approached a survey crew. The old man asked the
crew chief “do you still use a chain. The crew chief replied “sure whenever we
get the truck stuck”.
Which makes
me ask just how wide is a point?
