Contour plots

The command is twoway contour.

Or pull down the Graphics menu.

Either way, you can draw

. sysuse sandstone, clear
(Subsea elevation of Lamont sandstone in an area of Ohio)

. twoway contour depth northing easting, ccuts(7500(50)8000)  
     title("Subsea elevation of Lamont Sandstone, Ohio")

or this

. twoway contour depth northing easting, ccuts(7500(50)8000)
     title("Subsea elevation of Lamont Sandstone, Ohio")

Let us show you how this works.

Imagine you start with these data:

. clear

. set obs 100
Number of observations (_N) was 0, now 100.

. set seed 12345

. gen x = 2*runiform() - 1

. gen y = 2*runiform() - 1

. gen z = 1 - sqrt(x*x + y*y)

. drop if z < 0
(21 observations deleted)

. twoway contour z y x, interp(none) ccuts(0.1(0.1)0.9)

That’s almost an ordinary two-way scatter of 79 observations, except that it is of three variables, and the third variable is encoded by color.

Start with those data, specify that you want Shepard interpolation, and you get this:

. twoway contour z y x, interp(shepard) ccuts(0.1(0.1)0.9)

Or, specify that you want the default thin-plate spline, and you get this:

. twoway contour z y x, ccuts(0.1(0.1)0.9)