Add comment to clafify aggregation resolution limit

2018-02-01 10:26:52 +01:00
parent 80604b739a
commit 2e7784ddf2
1 changed files with 2 additions and 0 deletions
@@ -131,6 +131,8 @@ const dimensionDefs = ctx => {
 // This is equivalent to `${256/ctx.res}*CDB_XYZ_Resolution(CDB_ZoomFromScale(!scale_denominator!))`
 // This is defined by the ctx.res parameter, which is the number of grid cells per tile linear dimension
 // (i.e. each tile is divided into ctx.res*ctx.res cells).
+// We limit the the minimum resolution to avoid division by zero problems. The limit used is
+// the pixel size of zoom level 30 (i.e. 1/2*(30+8) of the full earth web-mercator extent), which is about 0.15 mm.
 const gridResolution = ctx => {
    const minimumResolution = 2*Math.PI*6378137/Math.pow(2,38);
    return `GREATEST(${256*0.00028/ctx.res}*!scale_denominator!, ${minimumResolution})::double precision`;