You said — With GPSs these things are relatively easy. You should see some of the corn mazes they come up with.
—
I’m not exactly sure what you mean by using GPS with these things. If you mean using GPS (even from some expensive GPS device of maybe $500 or so) to make the patterns accurately on the ground, using GPS numbers to position key points in the pattern, no, that wouldn’t work.
You would only say so, never having used GPS for trying to actually map or pinpoint stuff on the ground. I have, and there is quite a bit of error in the GPS coordinates, if you’re wanting to really pinpoint things. It just doesn’t do that good of a job.
For example, the following is a chart of GPS errors which are introduced by various things.
Ionospheric effects — ± 5 meters
Shifts in the satellite orbits — ± 2.5 meter
Clock errors of the satellites’ clocks — ± 2 meter
Multipath effect — ± 1 meter
Tropospheric effects — ± 0.5 meter
Calculation- und rounding errors — ± 1 meter
Altogether this sums up to an error of ± 15 meters. With the SA still activated, the error was in the range of ± 100 Meter. Corrections by systems like WAAS and EGNOS, which mainly reduce ionospheric effects, but also improve orbits and clock errors, the overall error is reduced to approximately ± 3 - 5 meters.
[ http://www.kowoma.de/en/gps/errors.htm ]
My GPS device has WAAS built in... so I get the maximum correction possible.
—
The Wide Area Augmentation System (WAAS) is a form of differential GPS (DGPS) giving enhanced position accuracy developed primarily for aeronautical navigation but usable by other users. Each Wide Area Reference Station (WRS) provides correction data to a Wide Area Master Station (WMS), which computes a grid of correction data to be uplinked to a geostationary satellite (GEO) via a Ground Earth Station (GES) in the Ground Uplink System (GUS). The geostationary satellite transmits the correction data (and also navigation data) to the user on the L1 GPS navigation frequency (1575.42 MHz). The user GPS receiver uses the downlink WAAS data to correct received navigation data. The goal of WAAS is to obtain at least a 7-meter horizontal and vertical accuracy.
[ http://users.erols.com/dlwilson/gpswaas.htm ]
—
So, some people with their GPS devices are going to be in an area of ± 15 meters, while I (and others) with WAAS are going to have an error of ± 3 - 5 meters.
Therefore, even with a highly accurate GPS with WAAS and an error of ± 3 - 5 meters — is going to put you off a possible *30 FEET* one way or the other...
That’s *way too much error* for constructing anything accurate, like what you see...
I’ve tried using GPS with WAAS and mapping stuff on the ground. It doesn’t work too well. What GPS works well in doing is navigating, but not putting you down on the ground to a precise location, unless you think 30 feet is precise enough for you. For me, that’s not precise enough for mapping something (on paper) that is on the ground — or vice versa — putting something that is on paper — “on the ground”...
I agree with your assessment of using a GPS. You really nailed that down with facts and figures. I simply note that my GPS flutters anywhere from 5-30 feet of accuracy constantly. By the time I stomped down a square foot of wheat it would have changed its reading about ten times.
I’m not that conversant with GPS as I should be, but I do know PLS now use GPS with their Total Base Stations, and it when tied to a nearby reference may also provide mm to cm resolution.
GPS kept changing so much in the 90s that PLSs couldn;t keep up with the changes, ...$5k min per shot of equipment, then 6 mo to 2yrs later it was out of date. It’s stabilized quite a bit now, but unfortunately, I haven’t had access to a good PLS update on the technology in about a decade. The professionals I work with, though, do use it daily and their flavor of GPS is quite a bit more rigorous than the consumer flavor on your SUV.
Thx.
Excellent points.