Problem in superimposing PGA on topography

HI,
I want to plot PGA value on background of topography. I am using GMT 4.5.5. I am able to create map but at the left side of map color is not covering whole area and getting problem. I am getting this problem in other maps also. Pls see attached JPG map.Is it aliasing in data??. How can i correct it? My script is:

#!/bin/sh
psbasemap -R/76/79.8/27.2/30.5 -JM15 -Ba1f1WSEN -K >delhi_new_1.ps

##GBF PGA
surface Delhi_SRTM1.xyz -Gdelhinew.grd -R76/80/27/30.5 -I15c/15c -T0.25 -V
grdgradient delhinew.grd -A45 -GGrad_delhinew -Ne0.6 -V

blockmean gbpga.dat -R/75/80/26/40 -I0.1m -V > gbpga.xyz
surface gbpga.xyz -Ggbpganew_1.grd -R76/80/27/30.5 -I15c/15c -T0.25 -V
grdimage gbpganew_1.grd -R/76/79.8/27.2/30.5 -JM15 -Ba1f1WSEN -IGrad_delhinew -Ccolor2.cpt -K -O >> delhi_new_1.ps

Waiting your response pls.

RBS

I suspect your CPT is not covering the full range of your grid range.

Dear Dr Wessel,Thanks a lot for pointing out the problem. I solved it by extending the range of cpt file below 0 (zero).
BUT I am worry that how negative values came in my grd file after running SURFACE. My input file ‘gbpga.DAT’ has values greater than 0 (zero) and my cpt file (color1.cpt) starts from 0 (zero). It means during interpolation and gridding process, negative values appear. Pls exactly let me know why it happened??? and how I can stop negative values during gridding process by SURFACE??

As to why it happens: Read about splines. Our surface paper explains what is being done. if you have no data near the end then the surface will take off in some direction based on the gradients in the area closest to the end. What to do? See purpose of -L as well as -M (well in more recent version that 4).