Subject: Re: mdvawv try advancing again too<br/>
Date: Aug 17, 2004 @ 15:44<br/>
Author: Ron McConnell (&quot;Ron McConnell&quot; &lt;rcmcc@...&gt;)<br/>
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<div id="ygrps-yiv-1829556232">&quot;aletheiak&quot; says,<br/>
&quot;... true mdvawv are in nad83<br/>
 nlat 39d19m16s80205 x wlong 77d43m10s14059<br/>
<br/>
What are the other lat/long values?<br/>
I didn&#39;t find them in a search of recent digests.<br/>
(Coulda looked right over them.)<br/>
<br/>
&quot;...distance of 79 feet & 6 inches...<br/>
&quot;...plus decimal places galore<br/>
but i don&#39;t know how far to trust them...<br/>
...limitations of the great circle distance calculator<br/>
or to some human error than to the surveyors art...&quot;<br/>
<br/>
Your instinct not to trust them is well justified.<br/>
There are calculators that output<br/>
12 or more decimal points for lat/long/distance<br/>
of which maybe only the first 4 or 5 are valid.<br/>
<br/>
You are obviously using a much better than average<br/>
great circle calculator.  The most common, modern<br/>
cosine spherical algorithm chokes long before<br/>
you get as close as 79&#39; 6&quot;.<br/>
The ancient haversine spherical formula works fine<br/>
for very close points.  There are at least<br/>
two ellipsoidal algorithms, one by Thaddeus Vincenty<br/>
and one by Emmanuel Sodano, that also work,<br/>
supposedly down to millimeters.<br/>
For such very short distances, ordinary plane<br/>
geometry works fine, too.  In any case<br/>
one must be very careful with the computer arithmetic<br/>
and the earth radius chosen<br/>
(there are many choices)<br/>
to preserve whatever accuracy one has<br/>
with the latitude and longitude input values.<br/>
Even with WAAS-capable GPS, getting that<br/>
0.1 second or last few feet are a challenge.<br/>
<br/>
Which great circle calculator(s) do you use?<br/>
<br/>
I have my own freeware great circle/geodesic<br/>
calculators on my web site (below)<br/>
with Vincenty, Sodano and spherical<br/>
(great circle and rhumb lines) algorithms.<br/>
I have limited the output distances<br/>
to 3 decimal places (km, miles, nmi)<br/>
since more than that is rarely justified<br/>
by the input latitude and longitude accuracy.<br/>
That is enough to compare results<br/>
among the different calculation methods<br/>
and datum choices for my own curiosity.<br/>
<br/>
The GCGC program also has the Vincenty direct/forward<br/>
algorithm where one can enter<br/>
<br/>
  lat1 & long1, azimuth 1-2 and distance 1-2<br/>
<br/>
and get out<br/>
<br/>
   lat2 & long2 and azimuth 2-1<br/>
<br/>
GCGC also calculates magnetic/compass bearings<br/>
from true azimuths using the magnetic declination/variation<br/>
offsets from World Magnetic Model<br/>
for the given lat1/long1 and lat2/long2.<br/>
<br/>
It would an easy and quick matter to output<br/>
more decimals, and units in feet, inches, chains, ...<br/>
for comparing such things as official marker locations<br/>
versus actual boundary points<br/>
if anyone is interested.<br/>
<br/>
Cheers, 73,<br/>
<br/>
Ron McC.<br/>
<a rel="nofollow" target="_blank" href="mailto:w2iol@...">w2iol@...</a><br/>
<br/>
Ronald C. McConnell, PhD<br/>
<br/>
 WGS-84: N 40º 46&#39; 57.6&quot;  +/-0.1&quot;<br/>
         W 74º 41&#39; 22.1&quot;  +/-0.1&quot;<br/>
         FN20ps.77GU31 +/-<br/>
         V +5058.3438  H +1504.2531<br/>
<br/>
 <a rel="nofollow" target="_blank" href="http://home.earthlink.net/~rcmcc">http://home.earthlink.net/~rcmcc</a> <br/>
<br/>
If a GPS receiver is misplaced,<br/>
but it is turned on and has a lock<br/>
on four or more satellites,<br/>
is it lost?</div>