Re: Trying to locate a property marker using GPS
- From: ixtok <ixtok72450@xxxxxxxxx>
- Date: Sat, 29 Jul 2006 08:33:41 GMT
On Wed, 26 Jul 2006 06:38:38 +1000, Chris Smolinski <cps@xxxxxxxxxxxxxxxxxxx> wrote:
I've been trying to locate a concrete property marker for some time now,
without luck, and wanted to see if I was going about it the right way.
According to the survey for my lot, there is supposed to be a concrete
marker at one of the back corners (I have a rather odd shaped 2 acre lot
with 9 sides). Unfortunately this marker is somewhere in the woods, in
an area filled with thick brambles, thorns, vines, etc. I'm trying to
clear out the brush, but want to know where I'm entitled to clear once I
start getting close to the property lines.
The survey shows the marker as being S 54 deg, 22', 0" E, 298.44 ft
from another corner point, which is close to my house and known.
So, I had the (maybe not so) bright idea of using a GPS receiver to try
to locate about where the marker should be. I understand that off the
shelf GPS receivers are not suitable for surveying, but my goal was not
to get a precise measurement, but within say 10 or 15 ft, so that I
would have a chance of finding the concrete marker. I'm using a GPS
receiver with a USB interface and WAAS, and some software I wrote to
average readings over several minutes, with a laptop. The first thing I
learned is that trees and leaves are not kind to GPS signals, and the
second thing is that trampling through the woods with a laptop and GPS
dongle isn't exactly fun. This might have to wait until fall when the
leaves are off the trees.
The survey has a arrow pointing North with the notation Mag North
2/1976, which I am assuming means that all of the bearings are
referenced to magnetic and not true north? From what I could find out,
for my location (76 deg 58 min East and 39 deg 42 min North) in 1976,
mangetic north was 8 degrees and 46 minutes West of true North. So the
bearing from the one point to the concrete marker (S 54 deg, 22', 0" E)
is really S 63 deg 8' 0" E, ref from true north, correct?
Since I don't have an absolute lon/lat location for any point on the
survey, I decided to take a GPS reading at the known point (OK, which
will introduce more error into my process) and then compute the lon/lat
location for the concrete marker. I ended up with an average position of
76 deg, 58 min, 28.066 sec W, 39 deg, 42 min, 15.960 sec N.
My understanding is that one second of latitude is 101.25 ft, and at my
latitude, one second of longitude is 78.1 ft.
Using simple trig, the 298.44 ft at that angle, the concrete marker,
bearing S 63 deg 8' 0" E is 266.2 ft east and 134.9 ft south of the
other point, or 3.408 seconds east and 1.332 seconds south of the point
at the other end of the line (76 deg, 58 min, 28.066 sec W, 39 deg, 42
min, 15.960 sec N), or located at 76 deg, 58 min, 24.658 sec W, 39 deg,
42 min, 14.628 sec N)
Any mistakes or bad assumptions with my above process? Needless to say
I didn't find the concrete marker. Before I try again, I want to make
sure I'm at least doing the calculations correctly, so I'm looking in
the right area. My hope is that with sufficient averaging I can get a
GPS location within 20 or so ft, which will let me find the marker.
Assuming this fails... what's the going rate for hiring a surveyor to
mark my property lines? ;-)
I think you are on the right track. The declination is about right. If I were doing it I would use a compass bearing of 117 deg and pace 300 feet.
If I wanted to play with a hand held GPS such as my simple Garmin Geko, I would set the position format to UTM instead of Deg.MMSS so that I would be using using the Transverse Mercator Coordinate System uning meters, take a position on the known corner as a waypoint like you did, use the "Project Waypoint" feature in the GPS reciever to calculate a coordinate for your missing corner using a bearing (azimuth) of 116 deg 52 minutes and a distance of 90 meters (rounded off). I think that the rest of your calculations are in the ball park.
Mark
.
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