Lesson 3a: The National Map Viewer Interface and Data Content.
The National Map viewer is a primary access point for the 'visualization' of geospatial
data that composes The National Map and functions as a tool to enable use of the data in a web
environment.
In this lesson, we will provide the user with an overview and demonstration of the interface
and data content of The National Map Viewer.
By the end of this lesson, you will be able to view National Map data via cached basemaps
and selectable layers, navigate the map, adjust transparency and viewing order of layers,
and view descriptions and metadata for National Map layers.
A subsequent lesson will show how to utilize the viewer’s tools to identify features,
perform queries, and acquire measurements, elevation profiles and spot heights.
Additional videos to show how to perform advanced functions will also be available.
The National Map viewer serves as a window into freely available base map data that is
consistent on a national level, and provides resources to access the data through a variety
of products and services.
To access the viewer, launch a web browser and navigate to The National Map launch page.
Once at this site, click the link for ‘TNM Viewer’.
For help at any time, click the “Link to Help” hyperlink.
First we’ll discuss basic navigation in the viewer.
Use the plus and minus buttons to zoom the current map view in or out.
You can also use the mouse scroll wheel to zoom in or out at the cursor location.
Or hold the shift button while clicking and dragging the cursor to zoom in to an area.
The Home button returns to the default map view.
Clicking on the ‘Basemap Gallery’ icon in the toolbar at the upper right displays
all of the base maps configured for this application.
The USGS basemaps shown provide cached background base maps for viewing the combined layers
in a single cached service to offer the user the simplicity of single-service base maps
with the richness of combined data sources.
The USGS basemaps are:
USGS Topo Base Map
USGS Imagery Topo
USGS Imagery Only
USGS Shaded Relief Only
USGS Hydro Cached
Other non-USGS basemaps are also provided for greater flexibility and convenience.
We will now zoom in to an area, to see the basemaps in more detail.”
The default basemap is the “USGS Topo Base Map”.
The USGS Topo Base Map displays a shaded relief topographic background with vector features
to provide complete base map visualization.
USGS Imagery Topo displays an imagery base map combined with vector features (including
contours) and is cartographically styled after the US Topo product.
USGS Imagery Only shows an “imagery only” base map suitable for overlaying custom feature
overlays.
USGS Shaded Relief displays a hill shade only base map suitable for overlaying custom feature
overlays.
USGS Hydro Cached displays hydrographic features associated with the National Hydrography Dataset
(NHD).
Now, let’s take a look at the Layer List.
The Layer List provides a list of operational layers and their symbols, allowing you to
turn individual layers on and off.
Each layer in the list has a check box that allows you to easily control its visibility.
Layers having expansion arrows indicate that they contain sublayers or subtypes.
The order in which layers appear in this window corresponds to the layer order in the map.
Clicking on the ellipsis to the right to the layer name layer provides additional options,
including the ability to view a description or metadata for that layer, the ability to
adjust the transparency of the layer in the map window, or to adjust the viewing order
for layers in the map.
The ‘Control All Layers’ icon on the upper right of the Layer List window controls the
behavior of all of the layers in the list.
Note that if you are using Internet Explorer the “control all layers” icon below does
not appear.
Click the icon to open the menu which allows you turn on or off all the layers, or expand
or collapse all the layers.
Alternatively, you can use keyboard shortcuts to do so.
Press and hold the Control button on the keyboard while clicking the check box of the layer
to turn on or off the layers in the same level, or press and hold the Control button while
clicking the arrow to expand or collapse layers in the same level.
Layer names are grayed out in the list when zoomed out of their visible ranges, but become
available when in zoom range.
For example, we will check the box for the NHD layer, and as we zoom in to an area, you
can see that the National Hydrography Dataset layer name becomes black when at the zoom
level that the data in that layer becomes visible.
You may be curious about the differences between the National Hydrography Dataset layer and
the Hydro Cached layer.
The dynamic NHD layer provides query capabilities for identifying features.
The dynamic NHD layer also provides a more detailed large scale view with additional
symbology and is designed to work between 1:577,000 scale and below.
The Hydro Cached layer provides a fast hydrography overlay styled after US Topo.
This cached tile service supports visualization from 1:591 Million to 1:9,000 scale.
Essentially the Hydro cached layer is for rapid cartographic rendering and the dynamic
NHD layer is for more detailed querying about the hydrography.
Clicking the X icon in the upper right corner of the Layer List window closes it.
Reopen the Layer List, and activate another layer by checking the box next to the layer
name, such as National Land Cover Database.
Expand that layer by clicking the arrow to the left of the layer.
In this example, the default activated layer is the NLCD 2011 dataset.
When a layer is turned on the feature types become available in the legend.
To view the legend, close the layer list, then click the Legend icon.
Close the legend window, and reopen the layer list.
Deactivate the land cover layer by unchecking the box.
Collapse the layer by clicking the arrow.
In this lesson, we discussed how to view National Map data via cached basemaps and selectable
layers, navigate the map, adjust transparency and viewing order of layers, and view metadata
for National Map layers.
In future lessons, you will learn how to utilize the viewer’s other tools to identify features,
perform queries, and acquire measurements, elevation profiles and spot heights, and also
perform advanced functions.