I know the "autobase" function is not real prevalent in land surveying but it is quite handy on construction sites using machine control. The trimble sps 851 bases that we use have an autobase function that allows it to remember where it was yesterday or last week and start itself automatically when you turn it on. I don't have to be there at the start of each day just so the dozers can work.
The way I understand it, the base receiver remembers a list of the last points it was on and the coordinates, HI, ect for that point. I don't know how many points it keeps in memory but it seems to be about 6-8 points.
If someone turns on a base while the satellites are in a poor geometry, high pdop, the position of the receiver may be off just enough that the base thinks it is not on the same point and it will not start like usual. It will show "Autobase Failed" on the display. Then I have to come and start it with the data collector. I don't know what distance it requires for it to assume the point data from the day before, I am guessing it is less than 10 feet. This seems to happen more often on jobs where I actually get the lat/long from OPUS rather that just hitting the "here" button. I am starting to think that the difference between NAD83 and WGS84 lat/long may be reducing my margin of error causing the base not to start more often.
One job I am starting has about 3.7 surface feet difference between the two, 1.93 north and 3.21 east. The "here" shot I took to initially start the base is 5.6 feet from the NAD83 coordinates that OPUS gave me later. I am considering using the WGS84 lat/long in the data collector and then apply a shift in the false north and east or just calibrate to the correct NAD83 north and east coordinates just to give the base a better chance of starting on it's own each morning. There are plenty of other control points I can check in to just to make sure I'm not screwing something up.
My questions:
1) Does anyone actually know how many points the base will store in memory?
2) Does anyone know the tolerance distance or the distance from where the base initially thinks it is at each morning to where the coordinates say it should be before the base will recognize it's location?
3) When the base is first turned on while sitting on a new point, it gets an autonomous position first, then it will get a waas corrected position. I know these two positions will be slightly different because of the accuracy of each. In a perfect world, are these two the same or is there a shift built into the waas corrected position?
I am wishing there was a way on increasing the tolerance distance. I will never have two control points that I will let the construction crew use for a base that are 100 feet apart or less. I doubt that I would ever have any less than 500 feet apart. I don't see any need in having the tolerance set so tight that a high pdop or 3.7 feet of shift due to using NAD83 would cause the autobase to fail.
Thanks for listening.
James
