Notes and Suggestions for Botanical Garden Surveyors - Part 2
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.
|Palmtop Coordinates| |Temporary Points| |Checking Back In|
To simplify the data entry, BG-Map allows you to select your occupied point and your backsight point from a list of coordinates that you have stored in the palm-top. (Editors Note - This is a new feature that will be available to all users in BG-Map version 5.0 to be released in the summer of 1999).
This is a very important and recommended feature. No matter how many points of occupation you have, it is worthwhile to record them in the palmtop file. It greatly helps minimize the chance for error. In any event, if you have used the field book to record your azimuths when setting points, you will be able to easily check to be sure your coordinates are properly recorded. Compare the computed azimuth in the palmtop to that in the field book. Of course itís preferable to do this in the office before heading out to occupy points. It would therefore be advisable to scroll through the entered coordinate values or inverse between points for comparison to check for mistakes in data entry.
Once you have established your control network and begin to set temporary points, be sure to use fairly durable material. To put the word "temporary" into perspective, the word "permanent" (when applied to survey markers) means lasting at least 25 years in the ground. Any time less may be considered "temporary", so thereís plenty of latitude in materials, from wooden stakes to metal markers. The choice depends on your locale and needs. The major point to keep in mind is that these should serve your day-to-day convenience and be considered points to go back to later, if needed. Donít take "temporary" too literally and remove them after one use. They may come in handy much later and will always be recoverable from the coordinate values you have set. If you are concerned about mowers being damaged, drive the points at least flush and perhaps below grade. Frost heaves can be a problem for wood but rarely affect nails or spikes.
Checking Back In
At the end of every setup it is wise to "check back in", i.e. turn or measure to a previously known point. Generally a distance measurement will be redundant and any real problems with the setup will be revealed in an azimuth check. Many problems that occur in the process of a setup go undetected until this final check in. These problems range from instrument settling to loss of correct beginning azimuth, skewing results. That final sighting before breaking the setup can avoid a lot of grief in the office later, puzzling over incorrectly located points.
Generally an azimuth check will alert you to problems; however it depends entirely on the distance from the setup to the sight. A 1-minute variance from the expected reading at a point 500 feet away amounts to 0.15 feet in horizontal error. Thatís not much for plant locations and that is the maximum error at 500 feet. In most cases, you will be working on material much closer than that. By contrast a 1-degree variance amounts to 8.73 feet at 500 feet - absolutely too much error.
As stated before, sighting a permanent azimuth mark at the beginning of the setup provides a quick and simple check at the conclusion of the setup, without the need for a second person to give the sight. The instrument operator does it alone.
- TO BE CONTINUED -
Updated May 4, 1999
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Updated May 4, 1999