The Query function in Internet Map Server provides a way to select
map features based on a logical operation. For example, you can create
a query to select all the polygons in a watershed whose land use is classified
as agricultural. Running this query will do two things - it will highlight
all the agricultural polygons on the map, and it will return a table
containing all the records for those polygons.
A Query is made by creating a Query String, and then Executing it. Here is an example of a simple Query String:
LANDUSE = "Agricultural"
Here is an example of a slightly more complicated Query String:
LANDUSE = "Agricultural" AND AREA > 50.0
This query selects those polygons whose LANDUSE type is Agricultural
AND whose AREA is greater than 50 hectares.
You can see that queries are based on the Boolean logic operators AND,
OR, and NOT. The Booleans were a primitive tribe of programmers,
who ultimately produced the SQL language, and promptly went extinct.
The Internet Map Server provides a number of tools for building and
executing a query, as shown in the figures below. The three
input fields allow the user to select the desired Field from the
database, the Boolean
operator, and a Sample Value. Once an expression
is created, the Add
to Query String button will place the completed expression in
a window. Once an expression is in the window, the Execute button
will run the query.
Two important items:
- In order to query a particular data layer, that layer must be Active.
To make a layer active, check the Acive radio button in the Layers
menu on the right side of the screen.
- While a query string can be typed in manually, it is better to pick
the fields from the menus. This is because the query is sensitive to
spelling - "Agriculture" will not work if the record in the database
is called "Agricultural". The Get Samples button will retrieve the
values from a particular field and make them available in the drop-down
This screenshot shows a query that has been created and added to the
query string, it is now ready to Execute
This image shows
the results after executing a query. Note that the agricultural land
uses are highlighted in yellow, and that the
first few records are shown below the map.
Finally, it is common to extract data from a GIS and analyze it with
another program, such as Excel. The Save Attributes to Text File
link will place the selected records into a CSV
(comma-separated value) text file, which can be saved to disk and imported into other
The Query function is a powerful tool for retrieving information from
maps; the fact that this can be done interactively over the Internet
gives a remote user capabilities that were previously only available
in a desktop GIS environment.