Geospatial Buffer & Projection Lab

The tool starts with a center and a radius in kilometers. It asks Turf to build a spherical geographic buffer and measures the resulting polygon. Here we isolate the radius parameter on a local radial diagram: eight pixels represents one kilometer. Start at ten kilometers, with latitude zero so the source scale factor is one. This outline is an explanatory radius diagram, not the actual projected map polygon. Increase the selected radius from ten to twenty kilometers. The local outline radius doubles from eighty to one hundred sixty pixels on the fixed scale. The tool’s spherical buffer receives the doubled radius. At the equator, the separate inflated-radius proxy also receives the same radius because one divided by cosine of zero is one. The source still calculates actual area from the Turf polygon rather than substituting this diagram. The selected geographic radius is now twenty kilometers. Doubling radius does not mean doubling area; however this video does not claim an exact area from pi times radius squared. The page computes area and perimeter of a finite spherical polygon. Its precision control changes polygon resolution, and the model is not an ellipsoidal survey calculation. Hold selected radius at ten kilometers. The source’s local Mercator scale proxy is one divided by cosine of latitude. At latitude zero, the proxy factor is one. The gray outline represents the selected ten kilometer radius; the cyan outline represents the radius multiplied by the proxy. Both use the same eight pixels per kilometer explanatory scale. Increase latitude from zero to sixty degrees without changing the selected radius. The cyan radius follows ten divided by cosine of latitude. At sixty degrees, cosine is one half, so the inflated radius is twenty kilometers. The gray selected-radius reference remains ten kilometers. This shows the exact source proxy arithmetic on shared axes, not a literal map-projection rendering. The radius multiplier is now two. The actual page creates another geographic Turf buffer using this inflated radius. It does not compute an exact planar buffer in Web Mercator here. Its displayed area distortion is obtained from the two resulting polygon areas, so it is not asserted to equal the scale factor squared exactly. The source radius multiplier depends on signed latitude through cosine. At negative sixty degrees, one divided by cosine is two. The graph uses latitude on the horizontal axis from negative sixty to positive sixty degrees, and multiplier on the vertical axis from one to two. The selected radius remains ten kilometers throughout. Move signed latitude from negative sixty through zero to positive sixty degrees. The marker descends to multiplier one at the equator, then rises to two again. Cosine is an even function, so opposite latitudes have the same source proxy factor. The input changes the chosen geographic center; this graph compares source parameter values rather than an elapsed physical journey. At positive sixty degrees, the multiplier is again two, equal to its initial value at negative sixty degrees. The same symmetry applies to this ten kilometer inflated-radius expression. It does not promise identical real terrain, longitude geometry, ellipsoidal accuracy or exact projected polygon area. The video explains the actual tool’s arithmetic while preserving its map controls and geographic exports.