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Basic
GIS Tools - Exercise 3 ANSWERS
1.) What field
indicates the site active/inactive
status? SITE_STATU
2.) How many
monitors are
active? 21 - your select query
should look like this: "SITE_STATU" = 'A'
3.) Does this
method create redundant data? NO
Why
or why not? A layer in ArcMap
represents a GIS dataset. It is possible to have a dataset
represented in a map more than once. Also you can limit the
features from a dataset in an ArcMap layer. The layer selection
does this, creating another layer in ArcMap without copying the
original data. The new layer is limited to those features which
were selected. This is a type of definition query.
4.) If someone
else were to add a new active ozone monitor
location to the data, would it show up the next time you opened your
map? NO Why or why
not? The selection layer is
based on a set of selected features, which are then put in a new
layer. ArcMap is not cognizant of the processes used to create
the selection, only that a certain number of features were
selected. It is a static snapshot of features based on a query at
the time. The only way it will change is if a feature were
actually deleted from the
original dataset.
5.) If someone else were to add a new inactive ozone monitor
location to the data, would it show up the next time you opened your
map? YES Why or why
not? The definition query is
read every time the layer is refreshed (redrawn). So if a new
inactive monitor was added, the next time the layer was refreshed you
would see the new monitor.
Challenge In step 4
above, what would happen in your map if someone else were to change the
status of a monitor from active to inactive? It would actually show up in both layers -
because originally it was part of the selected set that created the
selection layer, and now it satisfies the definition query and would be
in that layer as well.
With that in mind, is it preferable to use a selection
layer or a definition query when
limiting a layer based on an attribute
field? It is always better to
use a definition query if your layer can be limited using an attribute
query. |
