The trick is to „select“ a map defined from surveys in thconfig with full path, including the name of highest survey, where that map is defined.
source
index.th
select polygon.map@index_survey
export map -layout-debug station-names ### to display station names too.
######where#######
survey index_survey
-title "Perlová jaskyňa"
input
2015.th
input
2016.th
equate 48@2015_survey
0@2016_survey
map polygon.map -projection plan
2015_survey
2016_survey
endmap
endsurvey