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- What is digital imagery?
Digital imagery is a computer compatible version of
an aerial photograph, satellite photo or other map image. The 2007 digital
aerial photography is an example of digital imagery. In this case, the
photography was taken from a high altitude airplane that flew over the
state of New Jersey. These photos were then scanned into the computer as
digital photos (raster images), making them available for use in a GIS.
In order to be useful in a GIS, digital imagery must be further processed
to correct for scale distortions that occur when imaging the earth from
the air. This correction is called orthogonal rectification
(or ortho-rectification) and is covered in greater detail below
(FAQ #5). The 2007 aerial photography
has a one-foot pixel resolution, compared to the one-meter (three-foot)
resolution of the 1995/97 photography, and a scale of 1:2,400 (1 inch =
200 feet), compared to 1:12,000 for the 1995/97 (1 inch = 1,000 feet) photography.
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- Where can I get the digital aerial photography of New Jersey?
Aerial photography of New Jersey for 1995/97, 2002, and 2007
can be downloaded free of charge from the
New Jersey Information Warehouse application, or can be purchased from the
USGS. Information
on ordering can be found at the New Jersey Geographic Information Network
(NJGIN). Aerial photography can also be viewed
online using the
NJ-GeoWeb application and zooming into a large scale (the aerial
photographs are scale dependent and do not appear until the scale is
1:50,000 or larger (i.e., 1:45,000).
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- Is there more recent digital aerial photography for the State?
The most current digital aerial photography that provides
statewide coverage is the 2007 imagery. The
New Jersey Office of GIS coordinated a
statewide overflight in 2007. These images are available
for download through the
NJGIN Information Warehouse. Because the resolution of the 2007 imagery
is much finer (one foot pixels as compared with one meter in 1995/97), file
sizes are much larger than for the 1995/97 imagery. Like the 1995/97 imagery,
the 2007 photos are color infra-red but naturals are also available.
Partial coverage of the State for 2000 is available through
the Delaware Valley Regional
Planning Commission (DVRPC).
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- What kind of computer and software do I need to view digital aerial photography?
The higher the quality of your computer the more satisfied you will be with the
images and the draw time. A computer with a minimum of 4 gigabytes of RAM is
recommended. Additional video RAM is helpful because these images are large (up to 8 Mb each).
To view the 2007, 2002, or 1995/97 aerial photography, software is needed that can view JPEG (*.jpg) or
MrSid (*.sid) files.
The NJ-GeoWeb application
is the easiest way to view the 2007, 2002 or the 1995/97 aerial photography. The photography
becomes available as you zoom in to an area (because of the size of the images,
it is not practical to draw them for large areas of the State). Also be aware that
the imagery will draw more quickly if you have a fast connection to the
Internet. If you have dial-up Internet service you will have to be patient while
the imagery downloads and draws.
A free GIS data viewer called
ArcGIS Explorer can be used to view digital imagery in a variety of formats, including the
2007, 2002 and the 1995/97 aerial photography. The ArcGIS Explorer software must first be
downloaded and installed on the user’s machine. For additional information, read ESRI's ArcGIS Explorer
Common Questions.
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- What are Digital Ortho Photo Quads (DOQs) and Digital Ortho Quarter Quads (DOQQs)?
These are terms
used to describe aerial photography in digital format that has been digitally
processed and transformed from image coordinates to real-world
coordinates. Orthogonal (ortho)
rectification makes corrections within a photograph so that the scale is
uniform throughout the resulting image. The digital ortho-photographs combine
the image characteristics of a photograph with the geometric qualities of a
map. Distances and locations from these images can be accurately measured, and
they can be used just like other GIS data layers.
The photographs are referred to as quarter-quads because they
show one-quarter the area of a standard USGS 1:24,000 scale quadrangle (or
“quad”). The scale of a quarter quad is 1:12,000. Note that the 2002 imagery
is not tiled by quadrangles or quarter quadrangles, but was produced as a
series of 5000 by 5000 foot tiles.
For more detailed information, view the full
metadata or the
USGS DOQ web site, or read the
USGS DOQ FAQs.
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- Why doesn't the color of the aerial photography look right?
It is important to know that the 2002 and the 1995/97 aerial
photography was done in color infrared (the photographic film was sensitive
to infrared radiation). Infrared
light enhances discrimination between natural and man-made objects. Vegetation
and water surfaces stand out from vacant land and physical improvements—buildings,
structures and the like. This is sometimes referred to as “false-color”
photography because the resulting images are not in “true-color.” You can recognize
this photography because some vegetation tends to be red or pink because it
reflects in the infrared bands.
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- What about image quality? How detailed are the digital orthophotos?
The 2002 photography has a scale of 1:2,400 (1 inch = 200 feet)
and a resolution (or pixel size) of one foot. As a comparison, the 1995/97 aerials
were produced at a scale of 1:12,000 (1 inch = 1000 feet) and at a one-meter resolution.
Therefore, the 2002 photography is much more detailed than that from 1995/97. This
greater detail results in larger digital files, and consequently the aerial extent
of individual tiles are much smaller (5000 x 5000 feet compared to standard USGS
quarter quads).
On the 1995/97 photography, you should be able to see features
on the ground that are the size of an automobile or larger. With the 2002 photography,
you should be able to easily identify automobiles and other objects that are smaller
than one meter in size. For example, using the 2002 imagery, it is possible to discern
smaller structures, highway markings, swimming pools, tennis courts, and even sidewalks
and foot paths.
New photography flown in the spring of 2002, having a one-foot
resolution, is available from the
NJGIN Information Warehouse.
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- What is a world file?
A world file is a
text file associated with an image that contains the information that
identifies the image’s geographic location and orientation.
This allows images to be correctly
registered and layered with other GIS data. When obtaining imagery, you will
want to obtain both the image file
(*.tif, .jpg, .sid) as well as the associated world file (*.tfw, *.jgw, *.sdw).
These are often bundled together in a compressed (zipped) file.
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- Why are there various image formats, such as TIFF, JPEG and MrSID?
Digital image
formats evolve over time and newer formats are able to compress imagery to
reduce file size and allow for fast image drawing. Most GIS software packages
can read a variety of image formats. TIFF, JPEG and MrSID are some of the most
common formats currently used in a GIS. MrSid images are recommended for NJDEP
staff because they display more quickly than TIFF or JPEG images.
TIFF (Tagged interchange (image) file format) is an
industry-standard raster data format and is generally uncompressed. Due to the
large file sizes, NJDEP does not
usually make data available in TIFF format.
JPEG is a compressed image format, and is the format
used for the 1995/97 aerial photography. This provides for smaller file sizes
than TIFF images though there is some loss of image quality.
MrSID (Multi-resolution Seamless Image Database) is a more recent image format
developed by
LizardTech Software, which produces highly compressed images with
little loss of image quality. The MrSID format is becoming popular and
many GIS software packages now support it. In addition to high compression, the
format draws more quickly than JPEG or TIFF imagery. LizardTech markets a product
called GeoExpress that can be used to compress images to create MrSid files. For more information see
the GeoExpress with MrSid
product page or the
GeoExpress Support page at the LizardTech site.
The 2002 aerial photography is available in MrSid format
and the 1995/97 aerial photography is available in both JPEG and MrSid format from the
New Jersey Information Warehouse. The
images for download are compressed files that will include the image file
(*.jpg or .sid) and the world file (*.jpw or *.sdw). Both files are needed for
use with GIS software.
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- Who do I contact if I have more questions about digital imagery that
aren't answered here?
For additional
information please call the NJDEP GIS Help Desk at 609-777-0672 or contact us via e-mail at
gisnet@dep.state.nj.us.
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