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Using the "C" data line for completely free coordinates in Star*net
At risk of beating a dead horse, I must ask a question touched on in various previous threads another way, regarding the use of the “C” data line in Star*net. The manual says this:
“Coordinates are normally entered as completely fixed so they wonÛªt be adjusted, or as completely free to be adjusted. However at times you may wish to enter coordinate values with specified standard error values so that the coordinates are not considered completely fixed, and will be allowed some movement. These entered standard errors might be standard deviations resulting of some previous adjustment, or possibly standard deviations supplied along with some GPS points provided by others. In all cases, standard errors are entered in the same ÛÏorderÛ as the coordinates.”
If one desires to have various coordinates completely free…i.e. no standard errors, or constraints on their position, why even enter them?
In one recent project, I “#’d” out all the C lines, except those I wanted to hold. The error factor of the adjustment changed by .001 (not sure why).So, the question is: is there any difference at all between a line that reads, for instance:
C xxxx.xxxx yyyy.yyyy zzzz.zzzz * * *,
and no line at all? -
1 Reply
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In sense, and most of the time, you are right. A free coordinate has no affect on the final adjustement.
On occassion you need that free coordinate in there as a “seed” to enable to adjustment to proceed. For instance, lets say you had a traverse running down a road the went from 1 to 2 to 3 to 4 ,5 6 , 7, 8, 9 & 10. You have coordinates you want to hold for 6 and 7. You are probably going to need to provide free seed coordinates for 1 and 2, just to enable the apriori first guess to happen.
In practice, you return from the field with raw angle/distance data and a file of field determined, unadjusted, coordinates. You load all those coordinates into your dat file along with the measurements and fix those you want to fix, free those you want to free. Set your listing file to show “Coordinate changes from entered provisionals”. When you run your adjustment, the free coordinate will change just a little and that portion of the listing give you the 411 on just how much your coordinates differ from unadjusted.
As an adjustment proceeds you might find that a coordinate you initially intended to hold isn’t valid and you need to free it up. And vice versa. So that function is there to facilitate that, too.
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