Index: /trunk/Ohana/src/addstar/src/ReadStarsSDSS.c
===================================================================
--- /trunk/Ohana/src/addstar/src/ReadStarsSDSS.c	(revision 20687)
+++ /trunk/Ohana/src/addstar/src/ReadStarsSDSS.c	(revision 20688)
@@ -71,4 +71,9 @@
   NAMED_PHOTCODE_AND_ZP (photcode[3], zeropt[3], "I_SDSS");
   NAMED_PHOTCODE_AND_ZP (photcode[4], zeropt[4], "Z_SDSS");
+
+  // XXXYYYZZZ SDSS tables have special flags for undefined/unmeasured values and errors:
+  // -9999 flags unmeasured values, and the corresponding error may or may not be meaningful
+  // -1000 flags errors that are not determined, even though the corresponding quantity is.
+  // These special values need to be trapped here to avoid averaging meaningful numbers with the flag values.
 
   // various header values needed to calculate per-star data below