...
206 static inline unsigned char terrainType(const MAPTILE * tile)
207 {
208    return terrainTypes[TileNumber_tile(tile->texture)];
209 }
...

...
if (terrainType(mapTile(tileX,tileY)) == TER_WATER)
...

static inline WZ_DECL_PURE MAPTILE *mapTile(SDWORD x, SDWORD y)
/* Return a pointer to the tile structure at x,y */
static inline WZ_DECL_PURE MAPTILE *mapTile(SDWORD x, SDWORD y)
{
	// Clamp x and y values to actual ones
	// Give one tile worth of leeway before asserting, for units/transporters coming in from off-map.
	ASSERT(x >= -1, "mapTile: x value is too small (%d,%d) in %dx%d",x,y,mapWidth,mapHeight);
	ASSERT(y >= -1, "mapTile: y value is too small (%d,%d) in %dx%d",x,y,mapWidth,mapHeight);
	x = (x < 0 ? 0 : x);
	y = (y < 0 ? 0 : y);
	ASSERT(x < mapWidth + 1, "mapTile: x value is too big (%d,%d) in %dx%d",x,y,mapWidth,mapHeight);
	ASSERT(y < mapHeight + 1, "mapTile: y value is too big (%d,%d) in %dx%d",x,y,mapWidth,mapHeight);
	x = (x >= mapWidth ? mapWidth - 1 : x);
	y = (y >= mapHeight ? mapHeight - 1 : y);

	return &psMapTiles[x + (y * mapWidth)];
}

	x = (x >= mapWidth ? mapWidth - 1 : x);
	y = (y >= mapHeight ? mapHeight - 1 : y);

	return &psMapTiles[-1];