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<H1><A NAME="SECTION00630000000000000000"></A>
<A NAME="1472"></A>
<BR>
4.3 Color images
</H1>
<P>
Once a cpt file has been made it is relatively straightforward to generate
a color image of a gridded data. Here, we will extract a subset of the
global 30" DEM (data id 9) from USGS:
<P>
<PRE>
grdraster 9 -R-108/-103/35/40 -Gus.grd
</PRE>
<P>
Using <A NAME="tex2html262"
HREF="../grdinfo.html"><I><B>grdinfo</B></I></A><A NAME="3251"></A> we find that the data ranges from <IMG
WIDTH="16" HEIGHT="15" ALIGN="BOTTOM" BORDER="0"
SRC="img7.gif"
ALT="$\sim$">1000m to
<IMG
WIDTH="16" HEIGHT="15" ALIGN="BOTTOM" BORDER="0"
SRC="img7.gif"
ALT="$\sim$">4300m so we make a cpt file accordingly:
<P>
<PRE>
makecpt -Crainbow -T1000/5000/500 -Z >! topo.cpt
</PRE>
<P>
Color images are made with <A NAME="tex2html263"
HREF="../grdimage.html"><I><B>grdimage</B></I></A><A NAME="3260"></A> which takes the usual
common command options (by default the <B>-R</B> is taken from the data set)
and a cptfile; the main other options are
<P>
<BR><P></P>
<DIV ALIGN="CENTER"><A NAME="1760"></A>
<TABLE>
<CAPTION><STRONG>Table 4.3:</STRONG>
The main options in <A NAME="tex2html255"
HREF="../grdimage.html"><I><B>grdimage</B></I></A><A NAME="3273"></A></CAPTION>
<TR><TD>
<DIV ALIGN="CENTER"><TABLE CELLPADDING=3 BORDER="1">
<TR><TD ALIGN="CENTER" COLSPAN=1><FONT SIZE="-1">
<I>Option</I></FONT></TD>
<TD ALIGN="CENTER" COLSPAN=1><FONT SIZE="-1"> <I>Purpose</I></FONT></TD>
</TR>
<TR><TD ALIGN="LEFT"><FONT SIZE="-1">
</FONT><FONT SIZE="-1"><B>-E</B></FONT><FONT SIZE="-1"><I>dpi</I> </FONT></TD>
<TD ALIGN="LEFT"><FONT SIZE="-1"> Sets the desired resolution of the image [Default is data resolution] </FONT></TD>
</TR>
<TR><TD ALIGN="LEFT"><FONT SIZE="-1">
</FONT><FONT SIZE="-1"><B>-I</B></FONT><FONT SIZE="-1"><I>intenfile</I> </FONT></TD>
<TD ALIGN="LEFT"><FONT SIZE="-1"> Use artificial illumination using intensities from </FONT><FONT SIZE="-1"><I>intensfile</I> </FONT></TD>
</TR>
<TR><TD ALIGN="LEFT"><FONT SIZE="-1">
</FONT><FONT SIZE="-1"><B>-M</B> </FONT></TD>
<TD ALIGN="LEFT"><FONT SIZE="-1"> Force grayshade using the (television) YIQ conversion </FONT></TD>
</TR>
</TABLE><FONT SIZE="-1">
<A NAME="tbl:grdimage"></A></FONT></DIV></TD></TR>
</TABLE>
</DIV><P></P>
<BR>
<P>
We want to make a plain color map with a color bar superimposed above
the plot. We try
<P>
<PRE>
grdimage us.grd -JM6i -P -B2 -Ctopo.cpt -V -K >! topo.ps
psscale -D3i/8.5i/5i/0.25ih -Ctopo.cpt -I0.4 -B/:m: -O >> topo.ps
</PRE>
<P>
The plain color map lacks detail and fails to reveal the topographic
complexity of this Rocky Mountain region. What it needs is artificial
illumination. We want to simulate shading by a sun source in the east,
hence we derive the required intensities from the gradients of the
topography in the N90<IMG
WIDTH="11" HEIGHT="15" ALIGN="BOTTOM" BORDER="0"
SRC="img25.gif"
ALT="$^{o}$">E direction using <A NAME="tex2html264"
HREF="../grdgradient.html"><I><B>grdgradient</B></I></A><A NAME="3283"></A>. Other than the
required input and output filenames, the available options are
<A NAME="1502"></A>
<A NAME="1503"></A>
<P>
<BR><P></P>
<DIV ALIGN="CENTER"><A NAME="1763"></A>
<TABLE>
<CAPTION><STRONG>Table 4.4:</STRONG>
The <A NAME="tex2html258"
HREF="../grdgradient.html"><I><B>grdgradient</B></I></A><A NAME="3295"></A> options</CAPTION>
<TR><TD>
<DIV ALIGN="CENTER"><TABLE CELLPADDING=3 BORDER="1">
<TR><TD ALIGN="CENTER" COLSPAN=1><FONT SIZE="-1">
<I>Option</I></FONT></TD>
<TD ALIGN="CENTER" COLSPAN=1><FONT SIZE="-1"> <I>Purpose</I></FONT></TD>
</TR>
<TR><TD ALIGN="LEFT"><FONT SIZE="-1">
</FONT><FONT SIZE="-1"><B>-A</B></FONT><FONT SIZE="-1"><I>azimuth</I> </FONT></TD>
<TD ALIGN="LEFT"><FONT SIZE="-1"> Azimuthal direction for gradients </FONT></TD>
</TR>
<TR><TD ALIGN="LEFT"><FONT SIZE="-1">
</FONT><FONT SIZE="-1"><B>-M</B> </FONT></TD>
<TD ALIGN="LEFT"><FONT SIZE="-1"> Indicates that this is a geographic grid </FONT></TD>
</TR>
<TR><TD ALIGN="LEFT"><FONT SIZE="-1">
</FONT><FONT SIZE="-1"><B>-N</B>[</FONT><FONT SIZE="-1"><B>t</B><IMG
WIDTH="8" HEIGHT="31" ALIGN="MIDDLE" BORDER="0"
SRC="img1.gif"
ALT="$\vert$"></FONT><FONT SIZE="-1"><B>e</B>][</FONT><FONT SIZE="-1"><I>norm</I>[/</FONT><FONT SIZE="-1"><I>offset</I>]] </FONT></TD>
<TD ALIGN="LEFT"><FONT SIZE="-1"> Normalize gradients by </FONT><FONT SIZE="-1"><I>norm/offset</I> [= 1/0 by default]. </FONT></TD>
</TR>
<TR><TD ALIGN="LEFT"><FONT SIZE="-1">
</FONT></TD>
<TD ALIGN="LEFT"><FONT SIZE="-1"> Insert </FONT><FONT SIZE="-1"><B>t</B> to normalize by the <IMG
WIDTH="41" HEIGHT="35" ALIGN="MIDDLE" BORDER="0"
SRC="img69.gif"
ALT="$tan^{-1}$"> transformation. </FONT></TD>
</TR>
<TR><TD ALIGN="LEFT"><FONT SIZE="-1">
</FONT></TD>
<TD ALIGN="LEFT"><FONT SIZE="-1"> Insert </FONT><FONT SIZE="-1"><B>e</B> to normalize by the cumulative Laplace distribution. </FONT></TD>
</TR>
</TABLE><FONT SIZE="-1">
<A NAME="tbl:grdgradient"></A></FONT></DIV></TD></TR>
</TABLE>
</DIV><P></P>
<BR>
<P>
Figure <A HREF="node71.html#fig:GMT_atan">4.1</A> shows that raw slopes from bathymetry tend to be
far from normally distributed (left). By using the inverse tangent
transformation we can ensure a more uniform distribution (right).
The inverse tangent transform simply takes the raw slope estimate
(the <I>x</I> value at the arrow) and returns the corresponding inverse
tangent value (normalized to fall in the <IMG
WIDTH="24" HEIGHT="29" ALIGN="MIDDLE" BORDER="0"
SRC="img68.gif"
ALT="$\pm1$"> range; horizontal
arrow pointing to the <I>y</I>-value).
<P>
<P></P>
<DIV ALIGN="CENTER"><A NAME="fig:GMT_atan"></A><A NAME="3304"></A>
<TABLE>
<CAPTION ALIGN="BOTTOM"><STRONG>Figure 4.1:</STRONG>
How the inverse tangent operation works</CAPTION>
<TR><TD><IMG
WIDTH="633" HEIGHT="144" BORDER="0"
SRC="img70.gif"
ALT="\begin{figure}\centering\epsfig{figure=eps/GMT_atan.eps}\end{figure}"></TD></TR>
</TABLE>
</DIV><P></P>
<P>
Both <B>-Ne</B> and <B>-Nt</B> yield well behaved gradients. Personally,
we prefer to use the <B>-Ne</B> option; the value of
<I>norm</I> is subjective and you may experiment somewhat in the
0.5-5 range. For our case we choose
<P>
<PRE>
grdgradient us.grd -Ne0.8 -A100 -M -Gus_i.grd
</PRE>
<P>
Given the cpt file and the two gridded data sets we can
create the shaded relief image:
<P>
<PRE>
grdimage us.grd -Ius_i.grd -JM6i -P -B2 -Ctopo.cpt -K >! topo.ps
psscale -D3i/8.5i/5i/0.25ih -Ctopo.cpt -I0.4 -B/:m: -O >> topo.ps
</PRE>
<P>
<BR><HR>
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<ADDRESS>
Paul Wessel
2001-04-18
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