REVISED - 08/03/05 - PabloS

The longitude equations have been modified to draw almost perfect circles anywhere on the planet, except the polor regions. I've added the MS-Excel spreadsheet with the corrected version. It is attached to this Re-Post.


Original Message:

I'm trying to plot circles and squares in Google Earth. I'm using MS-Excel to compute the points on a circle, at different radii, given a distance in miles (statute). The real KML creation credit goes to the the author of the "KML Generator v0.1" website link, "ink_polaroid". I couldn't easily have done this without the website.

The circle math "looks" right, but it only looks properly proportional when the initial latitude and longitude are at the equator. As the initial Lat/Lon are placed farther north (or south), the circles become ovals and the horizontal distances become shorter. You can use Google Earth's measuring tool to see that happening. You can turn on the Lat/Lon grid to see it also.

My question is: What equation do I use to compute the points on a circle, given a certain radius from a given point?

Attached is a Zipped MS-Excel spreadsheet to "show my work".

To use the spreadsheet, place the radius of the smallest circle, in decimal miles, into cell A8.

In cells B8 and C8, place the center point Latitude and Longitude values respectively in decimal degrees, i.e., DD.dddddd format.

Copy the contents from cells B8 to C680 to the windows clipboard.

Click on the link in cell A9 then paste the clipboard contents into the coordinates window, click "a Path", then click "Go".

Very shortly you'll be prompted to click "right here". Doing so will have Google Earth display your handywork.

Any ideas to correct the equation of a circle to become circular for any given latitude?

Here you go
http://williams.best.vwh.net/avform.htm

Contains most formulae for spherical trig

Thanks for the information! I've seen the link before. It's a nice piece of information to have handy.

The longitude equations have been modified to draw almost perfect circles of almost any (reasonable) size, anywhere on the planet. The polor regions are still an exception due to the equations used.

I've added the MS-Excel spreadsheet with the corrected version. It is attached to this Re-Post.