\documentclass[border=10pt]{standalone}
\usepackage{asymptote}
\begin{document}
\begin{asy}[inline=true]
import graph;
import contour;
import palette;
defaultpen(fontsize(10pt));
size(14cm,8cm,IgnoreAspect);
pair xyMin=(1,74);
pair xyMax=(3,86);
real f(real x, real y) {return -2.051^3*1000/(2*3.1415*(2.99*10^2)^2)/(x^2*Cos(y)^2);}
int N=200;
int Levels=16;
defaultpen(1bp);
bounds range=bounds(-3,-0.10); // min(f(x,y)), max(f(x,y))
real[] Cvals=uniform(range.min,range.max,Levels);
guide[][] g=contour(f,xyMin,xyMax,Cvals,N,operator --);
pen[] Palette=Gradient(Levels,rgb(0.3,0.06,0.5),rgb(0.9,0.9,0.85));
for(int i=0;i<g.length-1;++i){
filldraw(g[i][0]--xyMin--(xyMax.x,xyMin.y)--xyMax--cycle,Palette[i],darkblue+0.2bp);
}
xaxis("$x$",BottomTop,xyMin.x,xyMax.x,darkblue+0.5bp,RightTicks(Step=0.2,step=0.05),above=true);
yaxis("$y$",LeftRight,xyMin.y,xyMax.y,darkblue+0.5bp,LeftTicks(Step=2,step=0.5),above=true);
palette("$f(x,y)$",range,point(SE)+(0.2,0),point(NE)+(0.3,0),Right,Palette,
PaletteTicks("$%+#0.1f$",N=Levels,olive+0.1bp));
\end{asy}
\end{document}
Source: TeX.SE