diff --git a/Toolbox/gsw_distance.m b/Toolbox/gsw_distance.m
index f6013b3..8abe41e 100644
--- a/Toolbox/gsw_distance.m
+++ b/Toolbox/gsw_distance.m
@@ -125,7 +125,7 @@
 a = (sin(0.5*dlat)).^2 + cos(lat(:,1:nla-1)*pi180).*cos(lat(:,2:nla)*pi180).*(sin(0.5*dlong)).^2;
 angles = 2 * atan2(sqrt(a),sqrt(1-a));
 
-p_mid = 0.5*(p(:,1:nla-1) + p(:,1:nla-1));
+p_mid = 0.5*(p(:,1:nla-1) + p(:,2:nla));
 lat_mid = 0.5*(lat(:,1:nla-1) + lat(:,2:nla));
 z = gsw_z_from_p(p_mid,lat_mid);        % Note. z is height and is negative
                                                             % in the ocean.