diff --git a/ArduCopter/config.h b/ArduCopter/config.h
index 628bf53e1c..13134f1e1a 100644
--- a/ArduCopter/config.h
+++ b/ArduCopter/config.h
@@ -300,8 +300,8 @@
 #define FS_GCS_ENABLED_CONTINUE_MISSION     2
 
 // pre-arm baro vs inertial nav max alt disparity
-#ifndef PREARM_MAX_ALT_DISPARITY_CMS
- # define PREARM_MAX_ALT_DISPARITY_CMS      100     // barometer and inertial nav altitude must be within this many centimeters
+#ifndef PREARM_MAX_ALT_DISPARITY_CM
+ # define PREARM_MAX_ALT_DISPARITY_CM       100     // barometer and inertial nav altitude must be within this many centimeters
 #endif
 
 // pre-arm check max velocity
diff --git a/ArduCopter/motors.pde b/ArduCopter/motors.pde
index 21f2efd734..65af49887a 100644
--- a/ArduCopter/motors.pde
+++ b/ArduCopter/motors.pde
@@ -536,7 +536,7 @@ static bool arm_checks(bool display_failure)
 
     // check Baro & inav alt are within 1m
     if ((g.arming_check == ARMING_CHECK_ALL) || (g.arming_check & ARMING_CHECK_BARO)) {
-        if(fabs(inertial_nav.get_altitude() - baro_alt) > PREARM_MAX_ALT_DISPARITY_CMS) {
+        if(fabs(inertial_nav.get_altitude() - baro_alt) > PREARM_MAX_ALT_DISPARITY_CM) {
             if (display_failure) {
                 gcs_send_text_P(SEVERITY_HIGH,PSTR("Arm: Alt disparity"));
             }