gps to distance

hi experts,

may i have your comment on my procedure to extrapouate a distance
delta over degree decimal coordinates along the longitude system?

i don.t require a high level of accuracy, but some would be nice.

let's start with some function used and a sample data set of san
diego.

goal: i an at petco park and have gps coordinates in decimal as
follows
lat 32.707
lon -117.155

i want to find what coordinate is 5 miles east of here. kind of like i
were building a directory lookup based on a user selecting 5 miles as
max bird's eye distance.

as longitude varies based on latitude i figure miles per degree
longitude like this:

lon : 69.09 * cos (lat in decimal degrees)
and get 58.1354 miles per degree longitude. since this is further than
the user requires, i need to segment this to find out how many miles
per minute like this:
m / min : 58.1354 / 60
and get 0.96892 almost one mile per minute of longitude.

now to get to how many minutes per mile i need to do this:
0.96892 * 1 / 60
and get 0.016149 this is how many minutes per mile in longitude. since
5 is max. it is multiplied by above.
0.016149 * 5 and get 0.080744

now to find 5 miles east of the park i add this to the coordinates
above.
-117.155 + 0.080744

now anything between 32.707 , -117.155 and 32.707 , -117.074256 is
what i wish to populate. of course this is a simplistic example and
doesn't draw a radius. just wondering if i an on the right track?
thanks for reading!

TATrader wrote:
(32.707, -117.155)SodanoD((51609.344),90)
32.70697064, -117.0691783

Ted

Ted Edwards wrote:

thank you for the quick response. it if that's the solution to the 5
miles east coordinate, then it looks like my attempt has a large
margin of error. i tried a web search and was unable to understand
SodanoD. may i know what i should look up on the web to help of
calculate my requirement well? always appreciated!

TATrader wrote:

You find plenty at
http://williams.best.vwh.net/avform.html
If you are only interested in short distances, William's flat earth
formulae will do well. Below are the results from using William's flat
earth formulae to convert the projection of my SodanoD back to distance
and direction. Distance in miles (5, 50 and 500), direction in degrees.
I used 45 degrees rather than 90 so as not specialize on either a
parallel or meridian.

Ò(32.707 ²117.155 Flat_rh(32.707,-117.155)SodanoD((5 50
500§mi),?45))÷Ômi 1
distance direction
5.0007061 45.02472912
50.07145818 45.24899352
508.053455 47.66249942

As you see, the error is quite small for short distances.

Most folk prefer Vincenty to Sodano because the formulae appear simpler.
If you are programming in a scalar language (e.g. BASIC) this *may*
be an advantage. I used APL2 for these.

Ted

Search for Vincenty instead of SodanoD. Vincenty's Direct Solution is
what you need.

BTW: For your test problem, Vincenty's Direct Solution yields 32.707N,
117.069W using WGS84

Vic