One of the characteristics of GIS is the possibility to combine information about several layers and datasets by using a geographic reference, e.g. a given point, a set of polygons or specific grid cells in a raster image. Queries are used to locate specific records from tables or places on the map based on the shared properties of layers. In GIS, a query can refer to two different functions. The first is a very basic GIS function that enables the user to interactively obtain information about a specific location in the GIS, using the Identify tool. The Identify tool is used to get information about specific pixel values within a raster image or to look at the attribute values of vector feature. In the example (Fig. 18), it can be seen that the location on the map which was clicked is somewhere in the commune of Düdingen and has an altitude of about 612 m. This information comes from the commune layer and the DEM that are currently in the ArcMap *.mxd.
The second, more advanced, type of query in GIS refers to the database
query. This type of query uses a structured query language (SQL), adopted from
database management systems, which allows the retrieval of information from the
database (Longley et al., 1999). In
ArcMap, the tool that incorporates SQL is the Select by Attributes (126.96.36.199.) tool. To go beyond the database
search, the Select by Location
(188.8.131.52.) tool can be used for geographic or locational queries of data.
Selections can also be conducted manually using the Select Features tool (184.108.40.206.). These three selection methods can
also be used in combination by changing the method of selection (Create new selection, Add to current
selection, Remove from current selection, Select from current selection) in
each respective dialog box. Any selection can be removed by clicking the Clear Selected Features button.
Fig. 18. The Identify tool dialog box displaying the results of a map query for all of the visible layers on the map. The results of the query show that the coordinates of the clicked location, which was located in the commune of Düdingen at an altitude of around 612m. The rest of the information in the dialog box comes directly from the attribute table of the commune layer.