Notes and Suggestions for Botanical Garden Surveyors - Part 3
ByWalt Dunlap, Mapping Specialist, The New York Botanical Garden
As a mapping specialist at The New York Botanical Garden and a professional land surveyor for the last 20 years, I am offering consulting services on land surveying to any garden that is mapping or planning to map their collections or garden.
|Total Station Care and Handling | |Total Station Pointing|
Total Station Care and Handling
If you use a total station, you have a substantial investment in its operation and maintenance. Although the manufacturer may stress its ruggedness, it is still a sensitive instrument subject to damage and misalignment. To help minimize the former, always keep at least one hand on it when placing it on the tripod and during the entire process of tightening it down. Many accidents can be avoided by the discipline of this simple and routine procedure. Many people who use dropped and repaired instruments swear that they are never as good as they were originally. It is truly catastrophic to the internal mechanisms.
Another good procedure to follow is not to tighten down the vertical and horizontal locks with too much force. This wears the threads unnecessarily and it also creates a tendency for the lock to slowly release prematurely, impairing precise pointing.
Any of the equipment pieces may be subject to misalignment over time, no matter how simple they seem. A good example is the line rod. It's pretty straightforward as a functional unit, but constant "harpooning" of the point will always result in the misalignment of the level bubble ("frog's eye") and affect your measurements. It can be an insidious mis-adjustment, not easily discovered by the instrument operator - especially if the direction of the bad alignment is always along the line of sight. Check the level bubble by shooting the same distance holding the rod at two positions 90-degrees apart.
Don't neglect to tighten all of the screws and bolts of the legs of the tripod from time to time. Consider the rule: "anything that can be adjusted will need to be". A wobbly tripod head can have severe effects on your accuracy.
There are commercial gizmos available to help with various other equipment adjustments. All I recommend is that you ask someone who uses one if they like the way it works and whether it was worth the usually extra expense as a specialty tool.
For adjusting the frog's eye, you need only create a plumb mark on the floor and on an 8' ceiling. Do this with a plumb bob, then secure the center of the rod between both points and adjust the three leveling screws against each other. If you have a spare point you may use it atop the rod and seat it in a drywall screw in the plumbed ceiling point.
There are simple tests to determine the basic adjustment of your instrument if - consult a survey text or your manual. Technical adjustments are always best done in a shop, but there are routine adjustments you can handle if you carefully follow the instructions of most owners' manuals. A biennial cleaning of a daily-used total station is normal and usually costs about $500 US. Of course if you begin to get suspicious results, it may be time for servicing. In some US states, boundary surveyors are mandated to compare distance results against a known established baseline semi-annually.
Total Station Pointing
The best recommendation for pointing the instrument is - Do it quickly. Dwelling on the target is usually less precise than a quick sight. It does take practice but, with time, it becomes easier. You will want to line up as low as you can on any hand-held sight, whether on a plumb bob or line rod. This eliminates as much as possible errors caused by the natural sway of such a target. Find the horizontal alignment first. Then elevate to the mirror or target. Fine tune the vertical as needed. Then take and record the shot.
- TO BE CONTINUED -
Updated May 17, 1999
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Updated May 17, 1999