Generate Random Training Points
The RBFLN neural network object
requires two sets of training points:
- one that defines the presence
of the objects or conditions to be predicted (i.e.,
mineral occurrences)
- a second that defines the
absence of these objects (i.e., locations where mineral
occurrences are known not to occur).
The two sets of points (locations)
are combined as training data.
This second type of data set is
often not readily available so this function offers one way of
approximating such a data set. The idea is to generate a set of
points in parts of the study area where there is a very low
probability of the object occurring.
One possibility for applying the
function is to generate a probability map (response theme) using
the weights of evidence or logistic regression methods and then
generate a set of random points in areas of very low probability.
This can be done as follows:
- Generate a response theme
using weights of evidence or logistic regression
following the steps described in Calculate
Response Theme.
- Select 'Generate Random
Training Points' from the SDM menu.
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- In the 'Non-deposit
Training Points' dialog, specify:
- the name of the
response theme generated in step 1
- the field containing
the probability values (in this example, either
(Arc-SDM Posterior Probability or LR Posterior
Probability)
- a threshold value:
all of the points generated will fall in areas
that have values less than this value.
In this example, the
threshold should be lower than the prior probability. The
prior probability can be found in the third column of the
last row of the weights table associated with the
response theme, or can be reported by clicking the  button
in the deposit training point theme properties dialog.
This dialog is displayed by clicking the 'Properties'
button in the 'Analysis Parameters' dialog.
- enter the number of
points to be generated. The default is the number
of points in the deposit training point set.
- Click OK.
- When prompted,
specify the file name and location for the new
point shapefile. A point theme is created and
added to the top of the table of contents of the
active view.
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General Application
The 'Generate Random Training
Points' function can use any grid theme in the active view. If a
floating point grid theme is selected, the threshold value must
be entered by the user. The minimum and maximum values in the
grid theme are reported as guidance and the default value is the
midpoint between these two.
| For example,
in this portion of the 'Non-deposit Training Points'
dialog, for the selected theme (not shown) the minimum
value is 0, the maximum is 1 and the default value is
0.5. |
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