diff --git a/src/p_setup.cpp b/src/p_setup.cpp
index e2d626936..292f3aa3a 100644
--- a/src/p_setup.cpp
+++ b/src/p_setup.cpp
@@ -2179,7 +2179,14 @@ void P_LoadLineDefs2 (MapData * map)
 
 		// convert the activation type
 		ld->activation = 1 << GET_SPAC(ld->flags);
-		if (ld->activation == SPAC_AnyCross) ld->activation = SPAC_Impact|SPAC_PCross;	// this is really PTouch
+		if (ld->activation == SPAC_AnyCross)
+		{ // this is really PTouch
+			ld->activation = SPAC_Impact | SPAC_PCross;
+		}
+		else if (ld->activation == SPAC_Impact)
+		{ // In non-UMDF maps, Impact implies PCross
+			ld->activation = SPAC_Impact | SPAC_PCross;
+		}
 		ld->flags &= ~ML_SPAC_MASK;
 	}
 	delete[] mldf;