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Public GIS Map Viewer: Layer Selection and Legend
When the map viewer is started, it defaults to a pre-selected set of layers. Some layers are on by default,
others are off. Additionaly, some layers are only visible within a certain zoom level, which is sometimes
necessary to keep the map legible while improving drawing speed and user response time. For example, parcel
outlines are not visible at the full zoom out level, because they would not be distinguishable at that extent
and if shown would appear as a large splotchy image.
Types of Layers
Working With Layers
Legends
Working With Legends
Types Of Layers
A map layer is a collection of geographic elements, representing the position and sometimes shape of
a group of similar items. These elements are commonly one of five broad types:
Points are simply locations with X and Y coordinates but no shape or size. Points are used
to represent locations of items such as manhole covers, fire hydrants, valves, switches, etc.
Lines can be straight or curved, and have length but no width (and thus no area). Lines are
commonly used to represent items such as street centerlines, water mains, railroads, topographical curves,
etc.
Polygons are used to represent geographic areas of various sizes and shapes. Layers such as
parcels, water bodies, voting wards, zoning districts, etc. are usually represented using polygons.
Annotation layers are simply geo-referenced text labels placed to show names, numbers, or
descriptions of locations or other layers.
Images are typically aerial pictures of an area, made up of multiple photographs taken from
aircraft using special cameras that record the location of the images as they are taken. These images
are then assembled into a composite image and orthorectified (aligned with known postions of georeferenced
objects on the ground) to produce a geographically-enabled picture that can be used by GIS users to verify
locations of other objects.
Working With Layers
Click the Layers tab located on the
left side of the browser window to access the controls for the various layers included in the map viewer.
For a quick overview of the layer controls, click the Layer Help button on that pane.
Layers are depicted using a familiar folder view, similar to working with files on a computer. Folders
represent layer categories. Clicking a folder icon will toggle the list of layers within that
category.
Clicking a gray box next to a layer name will toggle that layer on or off. If a check mark is shown,
that layer is currently visible. If a magnifying glass is shown, that layer will be visible once the
proper zoom level range is reached. A blank box means that layer is not currently visible.
Click the Refresh Map button to update the map display after you have toggled layers on or off.
You can also click the Auto Refresh checkbox to automatically update the map display each time
you toggle a layer on or off.
NOTE: Even though a layer may be visible, whether you see its
elements depends on the current zoom level and map extent area. For example, the city border layer
can be turned on, but if you are zoomed into the downtown area you won't see the border since it is
elsewhere on the map.
Legends
Legends show the symbology and color schemes used for the various layers currently visible. Some
layers may have multiple symbols and/or different colors to depict various categories within that
layer; for example, private hydrants appear differently than public hydrants, and different voting
wards are drawn using different colors.
Working With Legends
Click the Legend tab located on the left
side of the browser window to view the legend for the current group of visible layers. If you change the
list of layers shown by toggling different layers them on or off (see Working With Layers above),
the Legend will be updated once you click the Legend tab.
Note that visible annotation layers
are listed in the legend but have no legend symbol.
NOTE: As mentioned above, a layer may be
turned on but not be visible in the current map extent shown. Such layers will still appear in the
legend.
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