1. Aerial Vs Satellite Remote Sensing
Aerial Photography
Taking aerial photographs of the Earth's surface is a passive form of remote
sensing generated from cameras mounted on aircraft, satellites and other
spacecraft. The photographs are taken every 10 to 30 seconds as an aircraft
follows a systematic overlapping flight pattern at a fixed altitude. Each picture
slightly overlaps the preceding picture so that a stereoscopic (3-D) image of the
entire area can be produced and ground objects can be more easily interpreted
(note that a strict set of mathematical corrections are applied in an
aerotriangulation process prior to compilation to remove errors such as
atmospheric refraction, film shrinkage, and underwater refraction). NOAA's
primary aerial photographic product is a 9x9 inch color photograph, which is
usually exposed at scales from 1:10,000 to 1:50,000. NOAA photographers can
also capture images from select parts of the electromagnetic spectrum by using
various combinations of films and filters. The types of imagery they usually collect
include natural color, panchromatic (black-and-white), and false-color infrared
and black-and-white infrared photography.
Since the late 1930s, precision aerial photography has been the primary data
source for coastal survey maps, shoreline feature delineation maps, nautical
charts and other agency coastal geographical information systems. Unfortunately,
however, aerial photography has limitations in that it can only provide high
resolution spacial imagery when weather (e.g., cloud cover, sun angle) and
environmental (e.g., tidal) conditions are optimal.
Furthermore, the spectral sensitivity of aerial photography is limited to small
range from about near ultraviolet to near infrared. Therefore, Space agencies is
currently investigating existing and new remote sensing technology to augment
and/or replace conventional aerial photgrammetry. Unlike traditional aerial
photography, these techniques are able to capture images derived from a much
broader portion of the electromagnetic spectrum (from low-frequency radio
waves through gamma-ray regions of the spectrum) and in some cases are not
restricted by time of day, weather conditions, and other environmental
anomalies.
S K Diwakar

2. Aerial Vs Satellite Remote Sensing
Satellite Remote Sensing
Remote sensing is the science of remotely acquiring, processing,
interpreting and presenting spatial data for objects and environmental
processes using signals from a broad range within the electromagnetic
spectrum.
Remote sensing instruments are able to produce images of the physical
properties and characteristics of objects without being in physical contact
with them. Instead, this highly advanced technology forms images by
gathering, focusing, and recording reflected light from the sun, energy
emitted by the object itself, or reflected radar waves (which were emitted
by the satellite or other remote sensing devices). Therefore, remote
sensing can be further characterized as either “passive” or "active." Passive
remote sensing detects available (background) electromagnetic energy
from natural sources (such as sunlight), while active remote sensing,
depends on an artificial "light" source (such as radar) to illuminate the
scene.
NOAA uses digital photogrammetric work stations to produce numerous
remote sensing products, including shoreline vectors and maps, digital
elevation models, digital terrain models, airport layout diagrams, aviation
obstruction charts, and various other special use maps and products.
Furthermore, more advanced remote sensing technologies can be less
expensive than collecting the same type and quantity of data using aerial
photography and conventional ground survey techniques.
Satellite Imagery vs. Aerial Photography
Satellite imagery is an alternative to aerial photography but there is a lot
of confusion as to what is actually satellite and what is aerial photography.
Many people think that the data seen in Google Earth is taken from a
satellite when in reality it is high resolution digital aerial photography.
Similarly, many television shows, such as the BBC's Spooks, refers to their
S K Diwakar

3. Aerial Vs Satellite Remote Sensing
imagery of towns and cities as satellite data when it is actually aerial
data
photography supplied by Bluesky! So, what are the main differences
between the two and which would be most suited to your project?
Below is a list of benefits and weaknesses for both aerial photography and
satellite imagery. This information should help you make an informed decision
is
on the type most suited to your requirements.
Satellite imagery Aerial photography
Speed Speed
Satellites are capable of collecting Aerial photography used to be a
large amounts of data in relatively slow and time consuming process.
small amounts of time. A modern The amount of time taken to
satellite can be moved into position in capture an area depends greatly on
less than 3 days and can take the its size and shape. As airplanes fly
photographs quickly once locked on to back and forth in 'runs' a series of
an area. The size of these pictures are overlapping photographs are taken.
very large and allow the complete area The dawn of digital aerial
to be captured using less images and photography cameras has made the
therefore, in a shorter space of time. acquisition of airborne
This is necessary with satellites as, photography considerably quicker.
because of their limited numbers, the However, the newest cameras are
satellite may be required in a different making this faster still by
location very soon after. recording strips of data rather
than individual frames.
Level of detail Level of detail
Satellites generally reside several Aerial photography has the
hundred kilometers above the earth's distinct advantage of having the
surface. Although satellite imagery has lens closer to the subject, in this
improved greatly over the years it is case the land. By adjusting the
still lower resolution than aerial flying height of the aircraft
photography. High resolution satellite capturing the data it is possible to
imagery as high as 50cm per pixel is improve the detail that can be
readily available, up to 41cm in the case captured. Most aerial photography
photogra
of GeoEye-1 (however, the U.S.
1 was flown with a resolution of
Military requires resampling the between 50cm to 12.5cm per pixel.
imagery to 50cm for all customers not With the new technology of aerial
S K Diwakar

4. Aerial Vs Satellite Remote Sensing
explicitly granted a waiver by the U.S. photographic cameras it is not
Government). Military satellites more
t). unusual for newer imagery to be
than likely have a higher resolution but captured at 10cm per pixel. In
as yet this imagery has not become some cases this can be as low as
publically available. Most off-the-shelf
off 5cm or 2.5cm per pixel. Ho
However,
satellite imagery is between 250m and these ultra-high resolution
high
50cm in resolution. datasets consume enormous
amounts of storage space and
consists of many times the number
of photographs than lower
resolution datasets. Also, having a
lower flying height enables
airborne acquired multi-spectral
multi
and hyperspectral data to maintain
spectral
a higher resolution increasing its
potential uses.
Weather conditions Weather conditions
Both satellite imagery and aerial Although aerial photography can
photography can suffer from be affected by adverse weather
environmental conditions. Being higher conditions there is still the
up in the atmosphere means that
osphere possibility of photographing areas
satellites have more weather in thin or high level cloud which
n
conditions to cope with. Thin cloud that might stop the use of satellites.
may not stop aerial photography can This has a small bearing on the
still have a large effect on the quality quality of the final imagery and can
of satellite imagery. Due to its position normally be rectified during the
in orbit, and other requirements for post-processing stage. There are
processing
different areas the satellite may be
reas also far more planes available for
needed for another, clearer area. In taking aerial photo
photographs than
this case the window of opportunity for there are satellites so if one plane
photography may pass and it may be is needed where the weather is
some time before the satellite can be clearer then another may be
repositioned. It is worth noting that available to take its place. RADAR
RADAR data is not affected by is cloud penetrating so it is not
weather conditions so can be collected
itions affected by adverse weather.
at any time.
S K Diwakar

5. Aerial Vs Satellite Remote Sensing
Types of data Types of data
Many modern satellites can collect a Most aerial acquisition aircraft are
variety of data. These include standard fitted with a mount that allows a
photographic imagery, colour infrared number of different cameras or
and in some cases LiDAR thermal,
LiDAR, sensors to be attached to the
radar data. This variety of datasets
o same aircraft. This allows the
makes satellites very versatile. swapping of capture devices but
However, non-standard data sources
standard only when the plane is not airborne.
he
such as thermal, LiDAR, multi
multi-spectral Some aircraft can mount multiple
and hyperspectral imagery will likely be cameras or sensors and some of
expensive to capture or may not even the latest aerial cameras capture
be publically available. The main different types of data
problem with satellite dat types is
data simultaneously, such as standard
that when new or improved technology imagery, radar and colour infrared.
is released it is very difficult to Other datasets such as thermal
change the sensors and cameras in a imaging requires the flying to be
satellite but in a plane or helicopter it performed at night so a plane that
is simply a case of removing the old was capturing standard imagery
device and replacing it with the new during the day can be converted
one. during the evening and then sent
out again at night to capture
thermal data. Having a removable
sensor allows aerial acquisition to
keep abreast of the latest
developments in new technologies
and the cost of fitting these is
much lower than in satellites
orbiting many miles in orbit.
Location Location
The major advantage of satellite
satellit Location can cause a delay for
imagery is that the satellite can be aircraft acquisition teams. To
positioned to take imagery of anywhere guarantee that the survey can be
on the planet. It does not have to cross completed an acquisition team
borders or go through the many must remain on stand-by near to
stand
passport controls that could delay a the area to ensure that a weather
visiting acquision team. Being above the window is not missed. Luckily there
earth, it is more efficient for a are many aerial companies aro
around
S K Diwakar

6. Aerial Vs Satellite Remote Sensing
satellite to allow the earth to revolve the world who specialize in
below it than to physically move to the photographing their own country.
necessary location. Satellites still need This means that for a company in
to take into account the location of the Australia to acquire photography
sun to acquire visible light surveys so of the UK they only have to
many satellites try to remain in sun
sun- contact one of the companies that
synchronous orbit. specialize in data of that area.
Post-processing Post-processing
Imagery acquired by satellite usually Early aerial photography was made
consists of far fewer "shots" than that up of tens of thousands of
taken by aircraft. The extra distance individual photographs. When the
means that more area can be covered aerial photography industry
in one pass - at the deficit of detail. adopted digital cameras each
Satellites usually capture data in strips digital file was a copy of what was
(similar to a continua video of the
continual captured using analogue cameras.
area) and allow a larger amount of data Recent developments in aerial
to be acquired per digital file. This camera design have moved away
requires far less post-processing than
post from these traditional images and
if it were to capture individual "frame" moved towards the satellite's
images. method of capturing strips of
imagery instead. This has greatly
reduced the amount of post-
post
processing work required but, with
it's lower flying height, there are
still more images than with the
higher flying satellites.
S K Diwakar