diff --git a/include/prototypes.h b/include/prototypes.h
index 84614d08..d93c5c94 100644
--- a/include/prototypes.h
+++ b/include/prototypes.h
@@ -584,7 +584,7 @@ extern int ufindpfx(char *, int);
 extern int deltx(struct range *, coord);
 extern int delty(struct range *, coord);
 extern void radmap(int, int, int, int, double);
-extern void radmapupd(int, int, int, int, int, double);
+extern void rad_map_set(natid, int, int, int, int);
 /* rej.c */
 extern int setrel(natid, natid, int);
 extern int setcont(natid, natid, int);
diff --git a/src/lib/commands/navi.c b/src/lib/commands/navi.c
index c59a16ba..80588449 100644
--- a/src/lib/commands/navi.c
+++ b/src/lib/commands/navi.c
@@ -157,11 +157,9 @@ do_unit_move(struct emp_qelem *ulist, int *together,
 		cp = buf;
 	}
 	if (type == EF_SHIP) {
-	    radmapupd(player->cnum, leader->x, leader->y, leader->effic,
-		      (int)techfact(leader->tech,
-				    mchr[leader->type].m_vrnge),
-		      (mchr[leader->type].m_flags & M_SONAR)
-		      ? techfact(leader->tech, 1.0) : 0.0);
+	    rad_map_set(player->cnum, leader->x, leader->y, leader->effic,
+			(int)techfact(leader->tech,
+				      mchr[leader->type].m_vrnge));
 	}
 	if (cp == NULL || *cp == '\0')
 	    cp = &dirch[DIR_STOP];
diff --git a/src/lib/subs/radmap.c b/src/lib/subs/radmap.c
index 82aee6ee..6f7e625c 100644
--- a/src/lib/subs/radmap.c
+++ b/src/lib/subs/radmap.c
@@ -55,12 +55,6 @@ radmap(int cx, int cy, int eff, int range, double seesub)
     radmap2(player->cnum, cx, cy, eff, range, seesub, 1);
 }
 
-void
-radmapupd(int own, int cx, int cy, int eff, int range, double seesub)
-{
-    radmap2(own, cx, cy, eff, range, seesub, 0);
-}
-
 /* More dynamic world sized buffers.  We create 'em once, and then
  * never again.  No need to keep creating/tearing apart.  We may
  * want to do this in other places too where it doesn't matter. */
@@ -202,3 +196,35 @@ delty(struct range *r, coord y)
 	return y - r->ly;
     return y + WORLD_Y - r->ly;
 }
+
+/*
+ * Update OWNER's bmap for radar at CX,CY.
+ * EFF is the radar's efficiency, and RANGE its range at 100%
+ * efficiency.
+ */
+void
+rad_map_set(natid owner, int cx, int cy, int eff, int range)
+{
+    struct nstr_sect ns;
+    struct sctstr sect;
+    int changed = 0;
+    char ch;
+
+    range = (int)(range * (eff / 100.0));
+    if (range < 1)
+	range = 1;
+    snxtsct_dist(&ns, cx, cy, range);
+    while (nxtsct(&ns, &sect)) {
+	if (sect.sct_own == owner
+	    || sect.sct_type == SCT_WATER
+	    || sect.sct_type == SCT_MOUNT
+	    || sect.sct_type == SCT_WASTE
+	    || ns.curdist <= range / 3)
+	    ch = dchr[sect.sct_type].d_mnem;
+	else
+	    ch = '?';
+	changed += map_set(owner, ns.x, ns.y, ch, 0);
+    }
+    if (changed)
+	writemap(owner);
+}
diff --git a/src/lib/subs/shpsub.c b/src/lib/subs/shpsub.c
index f1c6b6e1..54dc821b 100644
--- a/src/lib/subs/shpsub.c
+++ b/src/lib/subs/shpsub.c
@@ -733,7 +733,6 @@ shp_nav_one_sector(struct emp_qelem *list, int dir, natid actor,
     int stopping = 0;
     double mobcost;
     double tech;		/* for mapping */
-    double tf;			/* for mapping */
     char dp[80];
     int navigate;
 
@@ -788,13 +787,9 @@ shp_nav_one_sector(struct emp_qelem *list, int dir, natid actor,
 	/* Now update the map for this ship */
 	tech = techfact(mlp->unit.ship.shp_tech,
 			((struct mchrstr *)mlp->chrp)->m_vrnge);
-	if (((struct mchrstr *)mlp->chrp)->m_flags & M_SONAR)
-	    tf = techfact(mlp->unit.ship.shp_tech, 1.0);
-	else
-	    tf = 0.0;
-	radmapupd(mlp->unit.ship.shp_own,
-		  mlp->unit.ship.shp_x, mlp->unit.ship.shp_y,
-		  (int)mlp->unit.ship.shp_effic, (int)tech, tf);
+	rad_map_set(mlp->unit.ship.shp_own,
+		    mlp->unit.ship.shp_x, mlp->unit.ship.shp_y,
+		    mlp->unit.ship.shp_effic, (int)tech);
     }
     if (QEMPTY(list))
 	return stopping;