Module talk:Location map/data/NATO

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Formulae after cropping

Latest comment: 10 months ago1 comment1 person in discussion

I find it difficult to understand the vertical stretch factor 575/324 in the formula for y introduced in this edit.

If I am not mistaken File:NATO Territory Map.svg is a derivative of File:North Atlantic Ocean laea ___location map.svg by cropping the lower border at 324/575 = 56.3478260869565%.

Imho this operation on a map with Lambert azimuthal equal-area projection cannot be compensated for by adding some vertical stretch factor but it requires the correction of the parameters ${\displaystyle px_{0}}$ , ${\displaystyle py_{0}}$ , ${\displaystyle dx}$  and ${\displaystyle dy}$ .

The upper right corner of the map remains at lat = 71.02192702284165 N; lon = 38.61651360858501 E and the projection center remains at lat = 35 N; lon = 35 W. The projection of the upper right corner remains at

${\displaystyle X_{UR}=0.97716044{\text{; }}Y_{UR}=0.82614817}$ 

However, the lower left corner moves to approximately lat = 14.35 N; lon = 96.55 W. Conversion of the lower left corner with the proj tool:

proj +proj=laea +lat_0=35 +lon_0=-35 +a=1 -f %.8f
-96.55 14.35
-0.97701214 -0.07076885

${\displaystyle X_{LL}=-0.97701214{\text{; }}Y_{LL}=-0.07076885}$ 

From these values the projection parameters can be computed as:

${\displaystyle px_{0}=-100{\frac {X_{LL}}{X_{UR}-X_{LL}}}=-100{\frac {-0.97701214}{0.97716044--0.97701214}}=49.9962}$ 

${\displaystyle py_{0}=100\times \left(1+{\frac {Y_{LL}}{Y_{UR}-Y_{LL}}}\right)=100\times \left(1+{\frac {-0.07076885}{0.82614817--0.07076885}}\right)=92.1098}$ 

${\displaystyle dx={\frac {1}{X_{UR}-X_{LL}}}={\frac {1}{0.97716044--0.97701214}}=0.5117}$ 

${\displaystyle dy={\frac {1}{Y_{UR}-Y_{LL}}}={\frac {1}{0.82614817--0.07076885}}=1.1149}$ 

Spischot (talk) 12:14, 2 October 2024 (UTC)Reply