Apostrophe notation for elevations in feet

[1ec5]

checkele() currently recognizes ft as the notation for feet within ele=*, converting it to meters:

OpenTopoMap/mapnik/tools/isolation.c

Line 65 in e4467cf

/* sanitize ele: substitute "," by ".", ignore "m", convert "ft" to meter, check if ele >> Mount Everest */

OpenTopoMap/mapnik/tools/isolation.c

Line 78 in e4467cf

if((strcmp(textrest,"ft")==0)||(strcmp(textrest," ft")==0)){ele=ele*0.3048;eleok=1;}

There is another notation for feet, ' (ASCII apostrophe), that is 40 times more common on length-related keys in general (including common keys such as height=*). The “Units” page on the wiki encourages the ##'##" notation and strongly discourages ft or in.

On the other hand, the ele=* documentation currently discourages unit specifications altogether. Despite this longstanding guideline, several major data consumers support both ft and ' for specifying elevations in feet. This proposal would codify that option for mappers. It doesn’t prefer one notation over the other, but perhaps you might consider adding support for ' for completeness.