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| GeoCommunity Mailing List |
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| Subject: | GISList: Satellite Maps - Summary |
| Date: |
03/17/2003 11:10:00 PM |
| From: |
Mohamed Eleiche |
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Hello Everyone,
I sent to the list some questions relating to the raster definitions,
Thanks for everyone replied,
and hereafter are what I got :
I need some precis definitions :
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1)What is the meaning of image accuracy & image resolution ,
and what is exactly the difference between them ?
2) Why high resolution satellite images are not suitable for
map production specially large scale ?
3) What is the difference between maps produced from
satellite images & those produced from aerial photos ?
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From: "Andrew Hoppin" <andrew@nspacelabs.com>
Sent: Wednesday, March 12, 2003 1:40 AM
People, it's been a while since I've done a lot of remote sensing work, so
feel free to correct me if I'm foggy on any of this, but, Mohamed, I think
this is basically what you want to know:
1) Accuracy can refer to Radiometric Accuracy- the ability of the sensor to
accurately identify different level of reflected light, or Geometric
Accuracy-- the accuracy of the location of a pixel in the recorded image of
a scene relative to its location in the corresponding "scene" on the ground.
Radiometric inaccuracies can be caused by light distortion in the
atmosphere, physical characteristics of the sensor, etc. Some of these
inaccuracies can be corrected with image processing software. Geometric
inaccuracies can be corrected by using precise ground control points (known
coordinate locations on the ground) to "tie" the image to these points,
whereupon the image processing software will "warp" the image to correct it.
Spatial Resolution refers to the fidelity of the sensor-- how many pixels
are recorded in the image per ground area. For example, a 10 meter
resolution means that each pixel of your image corresponds to 10 square
meters of area on the ground. Be careful though-- spectral resolution,
radiometric resolution, and temporal resolution are entirely different
concepts!
2) It can be difficult to use high resolution satellite images to create a
thematic map (e.g.: for land-use/land-cover) because there is so much
detail--- typically, in order to make a thematic map, you want to classify
the pixels in an image into groups and group adjacent similar pixels into
regions (polygons) representing a certain type of land-cover. It can be
easier to do this-- at least using automated techniques rather than direct
visual interpretation-- with a lower resolution satellite image. On the
other hand, digitizing a road
3) There is not necessarily any difference between maps from satellite
images and maps from aerial photos, though typically aerial photos are
higher resolution (more detail). The utility of the data from either source
depends on what your needs are, what type of sensor is on the platform
(airplane or satellite), how often and how recently the data is recorded,
whether the photography is recorded digitally or analog on film, etc.
TYPICALLY (but with many exceptions), available digital aerial photographs
are higher in spatial resolution (more detail) and narrower in radiometric
range than satellite imagery (aerial photography often records only
reflected light in the part of the electromagnetic spectrum visible to the
human eye, whereas satellite sensors typically record reflected light in at
least the near-infrared part of the spectrum as well as the visible).
Best regards,
Andrew Hoppin
****************************
Director of Client Services
N Space Labs, Inc.
www.NSpaceLabs.com
andrew@nspacelabs.com
212.219.0851 (office)
646.269.5602 (mobile)
594 Broadway, #611
NY, NY 10012
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From: "Bannerman Bruce" <BRUBAN@melbourne.vic.gov.au>
Sent: Wednesday, March 12, 2003 2:32 AM
For a precise defininition, you would be better off referring to a Remote
Sensing or Digital Image Processing text book. There are several good ones
available.
In answer to your questions:
1.
In simple terms, the term *image accuracy* can refer to:
- The positional accuracy of an image. That is, how close is a pixel to its
'true' location with reference to a spatial reference system (ellipsoid,
projection, datum, and possibly cartesian coordinate system).This would be
the usual context of the term.
- Image accuracy has also been used to refer to how closely the spectral
signature matches conditions present at the time the image was captured.
*Image resolution* normally refers to the size of individual pixels in an
image. For example Landsat TM is often quoted at 30m resolution and Spot 4
Panchromatic at 10m resolution.
In the case of the Spot 4 Panchromatic example, each pixel can be thought of
as cover
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