diff --git a/qpms/analysis.py b/qpms/analysis.py
index 4d79e4b..2b85b37 100644
--- a/qpms/analysis.py
+++ b/qpms/analysis.py
@@ -4,7 +4,7 @@ the ../misc directory.
 
 This needs to be kept in sync with the scripts and qpms/argproc.py.
 '''
-
+import math
 import numpy as np
 import qpms
 from qpms.qpms_c import Particle
@@ -79,11 +79,13 @@ class FiniteRectLatModesAnalysis:
                 y += 1
                 iris1.append(iri1)
         
-        # Mapping between ss particles and grid positions; 
-        positions = self.ss.positions.astype(np.float32) # cast to eliminate rounding errors
+        # Mapping between ss particles and grid positions
+        positions = self.ss.positions.astype(np.float32) 
+        # round to get rid of duplicities due to rounding errors
+        positions = positions.round(7-int(math.log(np.amax(abs(positions)),10)))
         xpositions = np.unique(positions[:,0]) 
-        assert(len(xpositions) == Nx)
         ypositions = np.unique(positions[:,1])
+        assert(len(xpositions) == Nx)
         assert(len(ypositions == Ny))
         # particle positions as integer indices
         posmap = np.empty((positions.shape[0],2), dtype=int)