[VHFcontesting] Calculating distances from grid squares

[David Pruett]

The note from KK6MC is a good overview about how distances between two 
grid squares can be calculated.  Once the distance is calculated, all 
that is needed is a little more math to apply the ARRL's proposed 
scoring rules to calculate the QSO point value.

People should not worry about how accurate their calculation is, or 
whether they make the calculation at all.  I am a volunteer log checker 
for the ARRL (ARRL 10M Contest) and all logs are fully scored by the 
logcheckers and there are no penalties if the claimed score in the log 
doesn't agree.

Of course, most people like to see their claimed score before they send 
in the log (to know approximately where they finished in regards to 
other stations and existing records).  My point here is that if your 
claimed score is off by a few percent due to differences in the distance 
algorithm you use vs. what the ARRL will use, there will be no negative 
impact.

73, Dave, K8CC

On 4/15/2016 12:16 AM, James Duffey wrote:
> Some have asked how the distance between the two grid squares are calculated.
>
> Here is one way that the distance between the six grid squares is calculated:
>
> The calculation starts by converting the 6 grid square center to a latitude and longitude. You can find the technique for doing that here:
>
> Tyson, N5JTY, Conversion Between Geodetic and Grid Locator Systems, QST January 1989, pp. 29-30, 43
>
> or there are several online calculators that will do that for you.
>
> Then, using high school trigonometry, the distance between the two points can be calculated:
>
> d = arccos(sin φ1 ⋅ sin φ2 + cos φ1 ⋅ cos φ2 ⋅ cos Δλ ) ⋅ R
>
> where d is the distance between the two lat-lon points, R is the radius of the earth, φ1 and φ2 are the latitudes of the two points, and Δλ is the difference in longitude between the two points. Angles are in radians.
>
> The earth is not quite a sphere and there are factors which can be added to that calculation to make it more precise. There is some dependence on the datum which is used in the calculation, but both these effects are probably small compared to the distance to the center of the 6 square grid that is ignored in the scoring calculation.
>
> I don’t know whether or not the ARRL will use this exact method, but it will use something not too different. I hope that this helps. - Duffey KK6MC
> --
> KK6MC
> James Duffey
> Cedar Crest NM
>
>
>
>
>
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