Index: trunk/doc/release.2015/ps1.detrend/detrend.tex
===================================================================
--- trunk/doc/release.2015/ps1.detrend/detrend.tex	(revision 40562)
+++ trunk/doc/release.2015/ps1.detrend/detrend.tex	(revision 40563)
@@ -1559,21 +1559,42 @@
 \section{Warping}
 \label{sec:warping}
-To provide a consistent and uniform set of coordinates for image
-combination (including stacking and differences), the individual
-mosaicked OTA images are projected onto common pixel grids, called
-tessellations.  A tessellation can contain any number of tangent plane
-projections, with those designed for single pointing surveys using
-only one, while the tessellation used for the $3\pi$ survey contains
-2643 tangent plane projection centers.  These ``projection cells'' are
-$4\times{}4$ degree fields spaced onto a set of centers that fully
-cover the sky.  They are arranged into rings of constant declination,
-and allowed to overlap as $|\delta|$ increases.  Each projection cell
-is further subdivided into $10\times{}10$ ``skycells'' with fixed
-$0.25"$ resolution pixels, and constant overlap regions between
-adjacent skycells of $60"$.  These skycells are the main image unit
-used for processing image data beyond the initial chip stage.  The
-coordinate system used for these images matches the parity of the sky,
-with north in the positive $y$ direction and east to the negative $x$
-direction.
+
+In order to perform image combination operations (stacking and
+differences), the individual OTA images are geometrically transformed
+to a set of images with a consistent and uniform relationship between
+sky coordinates and image pixels.  This warping operation 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 of
+``tessellations'' which specify how the sky is divided into individual
+images.  A single tessellation starts with a collection of projection
+centers distributed across the sky.  A grid of image pixels about each
+projection center corresponds to sky positions 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'.
+
+A tessellation can be defined for a limited region, with only a small
+number of projection centers (e.g., for processing the M31 region), or
+even a single projection center (e.g., for the Medium Deep fields).
+For the $3\pi$ survey, the tessellation contains projection centers
+covering the entire sky.  The version used to for the PV3 analysis is
+called the \ippmisc{RINGS.V3}.  This tessellation consists of 2643
+projection centers spaced every four degrees in DEC, with RA spacing
+of approximately four degrees, adjusted to ensure an integer number of
+equal-sized regions.  \ippmisc{RINGS.V3} uses a pixel scale of
+$0\farcs{}25$ per pixel.  The projections subdivided into a
+$10\times{}10$ grid of skycells, with an overlap region of
+60\arcsec\ between adjacent skycells to ensure that objects of modest
+size are not split on all images.  The coordinate system used for
+these images matches the parity of the sky, with north in the positive
+$y$ direction and east to the negative $x$ direction.
 
 After the detrending and photometry, the detection catalog for the