HP 48g Graphing Calculator - Manual (Page 322)

Plotting Functions of Two Variables
There are six different plot types that can be used to help you
visualize functions of two variables. Some of these display simulated
three-dimensional surfaces; other provide distinct two-dimensional
views of an underlying (but not displayed) “three-dimensional”
function.
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Sampling Grid
Functions of two independent variables need two inputs in order to
generate an output. The HP 48 uses a two-dimensional sampling grid
of points whose two coordinates provide the two inputs rec|uired.
Each of the six plot types that use functions of two variables allow you
to determine the size of the sampling grid. By default, it consists of 80
points—10 columns by
8 row^s. Increasing the number of points in the
sampling grid increases the time it takes to draw the plot—and the
detail with which the function is plotted.
However, for plots of functions of two variables, more detail does not
alw^ays translate into a more meaningful plot. Each combination of
function and plot type has its own built-in optimum sampling grid
size that is neither too small to properly reflect the function nor too
large to obscure the important aspect. You will probably need to
experiment a bit with the sampling grid dimensions when you first
plot a function.
Output Grid
The six plot types each transform the sampling grid into an output
grid, using the function to guide the transformation. However, each
plot type makes use of the sampling grid in different ways.
Three of them—Slopefield, Ps-Contour, and Gridmap—take each set
of sample coordinates and use the current equation to transform them
into a new two-dimensional output grid that enables you to visualize
the nature of the transforming equation. The plot you see is nothing
more than the two-dimensional output grid.
23-22 Plot Types