Index: trunk/doc/release.2015/ps1.datasystem/datasystem.tex
===================================================================
--- trunk/doc/release.2015/ps1.datasystem/datasystem.tex	(revision 40559)
+++ trunk/doc/release.2015/ps1.datasystem/datasystem.tex	(revision 40560)
@@ -779,5 +779,21 @@
   other related tessellations.}
 
-The \ippstage{warp} stage moves the data from a given exposure beyond
+The \ippstage{warp} stage transforms the image pixels from the regular
+grid laid out on the chips in the camera to a system of pixels with
+consistent geometry for a location on the sky.  The new image
+coordinate system is defined by one of a number ``tessellations'' of
+the sky.  Each tessellation specifies the way in which the sky is
+broken into individual images by defining a collection of projection
+centers.  For each projection center, positions on the sky are
+transformed to image pixels via a projection with a specified pixel
+scale and rotation.  In general, the pixel grid within the projection
+is defined as a simplified grid with the y-axis aligned to the
+Declination lines and no distortion terms.  The projection centers are
+typically separated by several degrees on the sky; for pixel scales
+appropriate to GPC1, the resulting collection of pixels would be
+unwieldy in terms of memory in the processing computer.  The pixel
+grid is thus subdivided into smaller sub-images called 'skycells'.  
+
+moves the data from a given exposure beyond
 away from being camera specific and towards a uniform sky oriented
 arrangement.  There are a number of ``tessellations'' defined and used