380
Chapter 13 Digital Landscaping
If you need to look up features on a topographic
map, there’s no need to wade through a map library
filled with topoquads of all sizes and scales. USGS
topo maps are made available online as DRGs. The
Libre Map Project is a great online resource for
1:24000 DRGs. Open your Web browser and go
to http://libremap.org, then select the Data tab.
From there, select your state. A new menu will appear and let you search for specific features (for
instance, selecting Florida for the state will allow
you to search for Walt Disney World as a feature,
and then download the DRG that contains Disney
World).
Use the Libre Map project to search for a prominent feature in your hometown or near your house or
school. Then download the DRG in the GeoTIFF file
format to your hard drive (and download the TFW
file too, to make sure the georeferencing information
comes with the map) and view it using a utility like
ArcGIS or AEJEE. Look over the DRG to find the feature you’re looking for. While you’re at it, examine the
contour lines near the selected feature and see how
they model the nearby elevation and terrain. Lastly, if
the DRG comes with a collar, look for the information
that tells you what the vertical datum is, when the
map was created, and when it was last photo-revised.
Hands-on Application 13.1
Digital Raster Graphics Online
This elevation (or vertical) difference between contour lines is called the
contour interval and is set up according to the constraints of the map and the
area being measured. In general, a wider contour interval is selected when
mapping more mountainous terrain (since there are numerous higher elevations) and a narrower contour interval is used when mapping flatter terrain
(since there are fewer changes in variation and more details can be mapped).
Small-scale maps (see Chapter 7) tend to use a wider contour interval because
they cover a larger geographic area and are presenting more generalized terrain information, whereas large-scale maps generally utilize a narrow contour
interval for the opposite reason (they show a smaller geographic area and
thus can present more detailed terrain information). DRGs (and their contour
lines) are frequently used as data sources within geospatial technology (such
as overlaying them with other layers in GIS). See Hands-on Application 13.1:
Digital Raster Graphics Online for a method of obtaining DRGs online.
While DRGs remain available (and many software programs give you
the capability of examining “seamless” digital topographic maps), the “next
generation” of digital topographic maps is the US Topo series. The US Topo
maps are delivered in 7.5 minute quad sections, but in GeoPDF file format
(see Chapter 7), which allows multiple layers of data to be stored on the
same map. These layers can then be turned on and off. US Topo maps include contours, recent transportation features (roads, railroads, airports),
transportation names, hydrography, boundaries, and orthophoto images, all
as separately accessible layers (see Figure 13.4 for an example of a US Topo
map) as well as the full collar of information (which can have its information be displayed or removed as a separate map layer). The USGS makes US
Topo maps available for free download (see Hands-on Application 13.2: US
Topo Maps as GeoPDFs for accessing and utilizing US Topo maps). Although
contour interval the
vertical difference
between contour lines
drawn on a map.
US Topo a digital
topographic map series
created by the USGS to
allow multiple layers
of data to be used on
a map in GeoPDF file
format.
Chapter 13 Digital Landscaping
If you need to look up features on a topographic
map, there’s no need to wade through a map library
filled with topoquads of all sizes and scales. USGS
topo maps are made available online as DRGs. The
Libre Map Project is a great online resource for
1:24000 DRGs. Open your Web browser and go
to http://libremap.org, then select the Data tab.
From there, select your state. A new menu will appear and let you search for specific features (for
instance, selecting Florida for the state will allow
you to search for Walt Disney World as a feature,
and then download the DRG that contains Disney
World).
Use the Libre Map project to search for a prominent feature in your hometown or near your house or
school. Then download the DRG in the GeoTIFF file
format to your hard drive (and download the TFW
file too, to make sure the georeferencing information
comes with the map) and view it using a utility like
ArcGIS or AEJEE. Look over the DRG to find the feature you’re looking for. While you’re at it, examine the
contour lines near the selected feature and see how
they model the nearby elevation and terrain. Lastly, if
the DRG comes with a collar, look for the information
that tells you what the vertical datum is, when the
map was created, and when it was last photo-revised.
Hands-on Application 13.1
Digital Raster Graphics Online
This elevation (or vertical) difference between contour lines is called the
contour interval and is set up according to the constraints of the map and the
area being measured. In general, a wider contour interval is selected when
mapping more mountainous terrain (since there are numerous higher elevations) and a narrower contour interval is used when mapping flatter terrain
(since there are fewer changes in variation and more details can be mapped).
Small-scale maps (see Chapter 7) tend to use a wider contour interval because
they cover a larger geographic area and are presenting more generalized terrain information, whereas large-scale maps generally utilize a narrow contour
interval for the opposite reason (they show a smaller geographic area and
thus can present more detailed terrain information). DRGs (and their contour
lines) are frequently used as data sources within geospatial technology (such
as overlaying them with other layers in GIS). See Hands-on Application 13.1:
Digital Raster Graphics Online for a method of obtaining DRGs online.
While DRGs remain available (and many software programs give you
the capability of examining “seamless” digital topographic maps), the “next
generation” of digital topographic maps is the US Topo series. The US Topo
maps are delivered in 7.5 minute quad sections, but in GeoPDF file format
(see Chapter 7), which allows multiple layers of data to be stored on the
same map. These layers can then be turned on and off. US Topo maps include contours, recent transportation features (roads, railroads, airports),
transportation names, hydrography, boundaries, and orthophoto images, all
as separately accessible layers (see Figure 13.4 for an example of a US Topo
map) as well as the full collar of information (which can have its information be displayed or removed as a separate map layer). The USGS makes US
Topo maps available for free download (see Hands-on Application 13.2: US
Topo Maps as GeoPDFs for accessing and utilizing US Topo maps). Although
contour interval the
vertical difference
between contour lines
drawn on a map.
US Topo a digital
topographic map series
created by the USGS to
allow multiple layers
of data to be used on
a map in GeoPDF file
format.