# Notes N9SSA's Miles Per Watt Calculator

Popularized by my friends at:

QST, January 2014 issue?
QST, June 2000 issue, Page 31
The Grid Locator and Field Hunting newsgroup

## Usage -

Note: This webpage uses JavaScript. Your browser must have JavaScript enabled to run this calculator.

Enter the longitude and latitude of two points in the standard +- format. For example: Chicago is Latitude: 41.880833, Longitude: -87.62785. Optionally, you can enter the gridsquare (2, 4 or 6 characters) and the calculator will automatically convert to Latitude and Longitude. Values for South and West are to be negative. Enter your power out in watts, and press the button!

You may also click Yes to have the calculator remember your FROM qth for the next time you visit.

If you want to see a map of your two points, press the Show Map button.

You can get coordinates for any site on earth through maps.google.com or coordinates of a callsign via BuckMaster. These, and other useful links are in the Tools section of the calculator. For ease of use with the calculator, I suggest you open tools with a RIGHT mouse-click, and "Open in New window", that way you can copy and paste back to the calculator window.

## Notes on Great Circle Distances -

The problem of determining the great circle distance on a sphere has been around for hundreds of years, as have both the Law of Cosines solution (not recommended) and the Haversine Formula:

Haversine Formula (from R.W. Sinnott, "Virtues of the Haversine", Sky and Telescope, vol. 68, no. 2, 1984, p. 159):

R = 6367000
dlon = lon2 - lon1
dlat = lat2 - lat1
a = (sin(dlat/2))^2 + cos(lat1) * cos(lat2) * (sin(dlon/2))^2
c = 2 * atan2( sqrt(a), sqrt(1-a) )
d = R * c

Where R is the radius of the earth. I used earthradius=6367000 meters. The earth actually varies from 6336 km to 6399 km.
This will give mathematically and computationally exact results. The intermediate result c is the great circle distance in radians. The great circle distance d will be in meters.

I based my distance calculator on the above formula. While not exact, it should give you results that will be within a couple of miles.

For further discussion, refer to http://www.movable-type.co.uk/scripts/gis-faq-5.1.html.

## Notes on Gridsquare Conversion -

This program will break down a gridsquare entry and convert it to Latitude and Longtitude. You can enter 2, 4 or 6 characters, and the calculator will select the center point of the grid (two chars), subgrid (4 chars), or sub-sub grid (6 characters).

Note that this program will takes lat/lon input literally. That is 52 degrees, 20 minutes is 52.3334. 52.3333 will resolve a different gridsquare than 52.3334. (No biggie). To experment, try 52.3333lat by 5lon and 52.3334lat by 5lon. Note how they come up with J022mh and J022mi respectively.

For further information on this, check out:
http://www.arrl.org/locate/gridinfo.html

## Maps -

In January of 2014 I rewrote the Mapping function to draw the signal path using a geodesic line utilizing Google Maps mapping API v3, based on the following code: https://developers.google.com/maps/documentation/javascript/examples/polyline-simple that interface to Google Maps.

While the maps are nice, it may not reflect the actual path traveled by your signal. Holland, Michigan to Perth Australia, for example. Did your signal travel west? Maybe it went east!

## Version History -

2.2 - Repaired mapgen.php
2.1 - Added Cookie to save home QTH location for repeat visitors
2.0 - Wrote companion program, mapgen.php, to display short path on Google Map with the Google Maps EZ interface to Google Maps
1.5 - Moved hosting from qsl.net to hoffswell.com
1.3 - Fixed remote broken links, including parc.xerox.com mapping
1.2 - Added input error checking, clean up code.