diff --git a/gp2rml/gp2rml.c b/gp2rml/gp2rml.c
index b9697c8..72563ee 100644
--- a/gp2rml/gp2rml.c
+++ b/gp2rml/gp2rml.c
@@ -189,6 +189,7 @@ static void usage(const char *name)
 	fprintf(stderr,
 "usage: %s [-s z_scale] z_clear[mm] xy_speed z_speed [file]\n\n"
 "  -s z_scale scale Z axis by z_scale (default: 1.0)\n"
+"  -t top     set the top elevation, in mm (default: auto-detect)\n"
 "  z_clear    clearance above the highest peak, in mm (must be > 0)\n"
 "             Unit (\"mm\") can optionally be specified\n"
 "  xy_speed   cutting speed, in mm/s\n"
@@ -206,13 +207,18 @@ int main(int argc, char *const *argv)
 	double p[3];
 	int c;
 
-	 while ((c = getopt(argc, argv, "s:")) != EOF)
+	 while ((c = getopt(argc, argv, "s:t:")) != EOF)
 		switch (c) {
 		case 's':
-			z_scale = strtod(optarg &end);
+			z_scale = strtod(optarg, &end);
 			if (*end)
 				usage(*argv);
 			break;
+		case 't':
+			z_max = strtod(optarg, &end);
+			if (*end && !strcmp(end, "mm"))
+				usage(*argv);
+			break;
 		default:
 			usage(*argv);
 		}