 Depending on the quality of aerial images used and the specific goals of the project, it
will probably be desirable to limit the minimum size of the features that are delineated. For example, poor image
resolution may preclude the interpretation of features smaller than some minimum size threshold. Other features, while
interpretable in the imagery, may simply be too small and therefore beyond the scope or goals of the desired map product.
To limit the size of the features that can be digitized in the habitat map, a minimum mapping unit (MMU) can be set in
Habitat Digitizer. Features must be larger than the MMU to be included in the habitat map. Set the MMU restriction by
selecting "Habitat Digitizer/Set Minimum Mapping Unit". If the view's Map and Distance units are set, a dialog will
appear showing the current MMU. Enter the desired numerical MMU into the text box and select "Apply New MMU". If a
satisfactory MMU has already been set, "Use Current MMU" will close the dialog without changing the MMU. If the area of
a newly digitized polygon is below the MMU, a message box will ask whether the polygon should be added to the theme. If
no MMU restriction is desired, "Habitat Digitizer/Set Minimum Mapping Unit/Turn off MMU" will allow digitizing polygons
with no size restriction.
It is possible to adjust the scale of the image files as they appear on the computer monitor. For example,
the scale of hard copy photographs used for mapping may be 1:48000, however the actual photo interpretation may be conducted
on the computer monitor while zoomed in on the scanned photographs at a much larger scale (e.g., 1:6000). It may be desirable
to conduct all polygon delineation at the same scale, such that all polygons have the same level of detail. Set the scale
restriction by selecting "Habitat Digitizer/Set Scale Restriction". Enter a number in the text box and select "Apply New
Restriction". If digitizing is attempted while a scale restriction is in place and the view is not at the specified scale,
a message box will offer to zoom the view to the proper scale. If "No" is selected, a polygon cannot be digitized. If a
scale restriction is not desired, use "Habitat Digitizer/Set Scale Restriction/Turn off Restriction" to allow digitizing at
any scale. |
