| SAS/SPECTRAVIEW Software User's Guide |
One
of the first steps in analyzing data is to examine all the data values, for
which the point cloud is ideal, for example, to show concentrations of data.
A point cloud displays response values as colored markers, showing individual
data points. The marker locations represent the plotted values of the X, Y,
and Z variables, and the color of each marker represents the range in which
that value resides as mapped in the data ramp.
You can display all response values in a point cloud,
or you can specify a subset by using the response histogram.
|
| Subsetting Response Values |
Using the response
histogram, you can specify a subset of response
values for a point cloud by setting a range of responses. The response histogram,
which appears at the bottom left of the interface when you request a point
cloud or an isosurface, displays the frequency distribution of data points
that share the same response value. The histogram may also include software-interpolated
values between the actual response values.
Response Histogram to Subset Point Cloud
![[IMAGE]](./images/01327156.gif)
There are actually two histograms:
- The upper one represents all the response values
in the data set. The positions of the lines are the minimum and maximum response
values.
- The lower one magnifies the portion of the data
that lies between the two lines. The histogram stacks are larger or smaller
relative to all the data between the lines. As you move the lines, you alter
the span of values being compared to each other, so the stacks in the lower
histogram change.
Flat histogram stacks do not necessarily indicate no
response data at that value. When you have a complete grid, the flat places
may simply indicate much fewer values compared to the frequency of values
represented by adjacent stacks. When you have only a few responses and they
are widely separated, the values interpolated between them by the software
may produce a flat stretch in the histogram.
-
Select [Tools], [Planes], then [Point cloud].
- To change the shape of the
marker, select one
from the panel of choices, which include a period, an X, an asterisk, a O,
and a +. The default is the asterisk.
- To change the size of the marker, move the slider
below the label MARKER SIZE. The default is a
moderate size marker. The larger the marker size, the more solid the point
cloud.
- To specify a subset of response values displayed
in the point cloud, use the response histogram located in the lower left of
the interface:
- For a minimum response value, position the cursor
on the left line, which represents the low-end values, then drag to the desired
lower limit.
- For a maximum response value, position the cursor
on the right line, which represents the high-end values, then drag to the
desired upper limit.
- To pan across a subsection of the values, place
the cursor inside the lines, then drag the cursor.
Note that you can modify subsetting criteria with
a point cloud displayed, but for a lot of data, you may want to turn off the
point cloud to speed up the process.
- Select [On].
The following point cloud displays a subset of response
values
from the EPA data set:
Point Cloud Displaying Location of High Sulfate Concentrations
![[IMAGE]](./images/01327157.gif)
Copyright 1999 by SAS Institute Inc., Cary, NC, USA. All rights reserved.
