diff --git a/src/create_surfaces.f90 b/src/create_surfaces.f90
index e8ac0a98a1e7d884b78f0f8bd5aefc94399ba79f..0f43b4c0a0e3d49dd12c2d47e6caf5fb86de9dfa 100644
--- a/src/create_surfaces.f90
+++ b/src/create_surfaces.f90
@@ -630,22 +630,23 @@ select case(surface_type)
            if (x(i).le.x1a) then
              zx=sp01
            elseif (x(i).le.x1b) then
-             zx=sp01-((x(i)-x1a)/(sp06/m)*sp06)
+             zx=sp01-((x(i)-x1a)/(-sp06/m))*sp06))
            elseif (x(i).le.x2a) then
              zx=sp01-sp06
            elseif (x(i).le.x2b) then
-             zx=sp01-((1.d0-((x(i)-x2a)/(sp06/m)))*sp06)
+             zx=sp01-(1.d0+((x(i)-x2a)/(-sp06/m))*sp06)))
+             !zx=sp01-((1.d0-((x(i)-x2a)/(sp06/m)))*sp06)
            else
              zx=sp01
            endif
            if (y(i).le.y1a) then
              z(i)=sp01
            elseif (y(i).le.y1b) then
-             z(i)=min(sp01-((y(i)-y1a)/(sp06/m)*sp06),zx)
+             z(i)=min(sp01-((y(i)-y1a)/(-sp06/m))*sp06)),zx)
            elseif (y(i).le.y2a) then
              z(i)=min(sp01-sp06,zx)
            elseif (y(i).le.y2b) then
-             z(i)=min(sp01-((1.d0-((y(i)-y2a)/(sp06/m)))*sp06),zx)
+             z(i)=min(sp01-(1.d0+((y(i)-y2a)/(-sp06/m))*sp06))),zx)
            else
              z(i)=sp01
            endif