FLT CREW RPTS CROSSING ILS HOLD LINE FOR RWY 31C AT MDW WITHOUT CLRNC DUE TO CONFUSION OVER PAINTED BOX INDICATING LINE IS FOR RWY 13L-31R. ARPT DIAGRAM IS SIMILARLY MARKED AND ALSO CONFUSING.

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ON TAXI OUT; INSTRUCTED BY GND TO HOLD SHORT OF RWY 31C ILS CRITICAL AREA. DE-ICING OPS ON OUR L ON TXWY E3; TFC AT TWO INTXNS ON OUR R AND INTXN OF RWY 13L/31R HOLD SHORT WITH RWY 13L/31R ILS CRITICAL TXWY MARKING ON OUR NOSE. WE PASSED WHAT I IDENTIFIED AS THE RWY 13L/31R ILS MARKING BECAUSE I OBSERVED THAT DESIGNATION; AND IT IS MARKED THAT WAY ON THE HOLD PAD DETAIL PAGE; AND FO STATED THAT THE ILS CRITICAL ZONE MARKING IN QUESTION WAS FOR RWY 31C -- THAT THERE WAS NO ZONE FOR RWY 31R. WE STOPPED AT THAT POINT AND ADVISED TOWER OUR NOSE WAS ACROSS THE LINE. THERE IS NO SIGNAGE OTHER THAN PAVEMENT MARKINGS (I.E. ABOVE GND RED SIGN). I STILL AM NOT SURE IF WE CROSSED THE ASSIGNED HOLD SHORT CRITICAL AREA OR NOT; BUT WE STILL ADVISED TOWER ('NO PROBLEM') THAT WE HAD. WX CONDITIONS WERE A MAJOR FACTOR IN THIS INCIDENT; AS WERE ARPT SIGNS (OR LACK THEREOF) AND INACCURATE CHARTS.SUPPLEMENTAL INFO FROM ACN 814698: THE ILS CRITICAL HOLD SHORT LINE IS NOT AS CLEARLY MARKED AS IT NEEDS TO BE IN THE CONDITIONS WHEN AN ILS CRITICAL AREA (HOLD SHORT LINE) BECOMES NECESSARY TO USE. BECAUSE OF THE POOR VISIBILITIES; NIGHT; -SN; OTHER ACFT IN THE AREA; WE TAXIED WITH OUR NOSE JUST PAST THE ILS CRITICAL AREA HOLD SHORT LINE. WE ADVISED ATC OF OUR POSITION AND HE CLEARED US FOR TAKEOFF. INSTALL IN-PAVEMENT LIGHTING AT MDW FOR THE AREA IN CONCERN.CALLBACK CONVERSATION WITH RPTR ACN 814682 REVEALED THE FOLLOWING INFO: THE RPTR WAS NOT FAMILIAR WITH THE PLACEMENT OF THE ILS HOLD LINE FOR RWY 31C AND THE LABEL JUST BEFORE CROSSING SEEMED TO INDICATE THAT IT WAS FOR RWY 31R. HE REALIZES AFTER THE FACT THAT RWY 31R DOES NOT HAVE AN ILS AND THAT THE SIGN HE WAS READING WAS UPSIDE DOWN FOR HIS DIRECTION OF TRAVEL. IN-GROUND LIGHTING WOULD MAKE THIS FOOLPROOF.