Lisa Greenbaum
GEOG 594, section 001
LAB ONE
Tuesday, February 13, 2007
Page 1
altitude reading for my point (OLD position):
78.13 m MSL NAD 1927 (Eastern US)
78.13 m MSL NAD 1984
(In order to set the coordinate units and the altitude units to meters, I had to change "System" from Lat/Long to UTM.)
81.28 m HAE NAD 1927 (Eastern US) (height with respect to reference elipsoid)
:
the difference between these two readings is:
the altitude is 3.15 meters higher when measured as height above reference elipsoid than height above MSL.
--------------------------------------------------------------------------------------------------the latitude at which I took data:
35 degrees 57.62890 minutes
i rounded this to 35.96 degrees.
cos(35.96) = .8094
one minute of longitude = 1855(.8094) meters = 1501.437 meters
---------------------------------------------------------------------------------------------------------
LATITUDE
computation based on the single "OLD" position
degrees and minutes
NAD27 (Eastern US)
35 degrees 57.62890 minutes N
WGS84 (alias NAD83)
35 degrees 57.63580 minutes N
Difference
.00690 minutes of latitude
In my area the representation in "Deg & Min" of the point in WGS84 (alias NAD83) is .0069 minutes NORTH
of the representation of that point in NAD27.
This corresponds to 1845(.0069) meters = 12.7305 meters.
-----------------------------------------------------------------------------------------------------------
UTM (in meters)
NAD27 (Eastern US)
3 981 257.53 m
WGS84
3 981 471.07 m
Difference
213.54 m
In my area (which is UTM Zone: Number 17, Letter N), the representation of the point in UTM coordinates
based on WGS84 is 213.54 meters NORTH of the representation of that point in UTM NAD27.
--------------------------------------------------------------------------------------------------LONGITUDE
computation based on the single "OLD" position
NAD27
78 degrees 58.59281 minutes
WGS84
78 degrees 58.57821 minutes
Difference
.01460 minutes
In my area, the representation in "Deg & Min" of the point in WGS84 (NAD83) is .01468 minutes EAST of the representation of that
point in NAD27.
This corresponds to 23 meters
(.01460)(1566.9185) meters in one minute of longitude at this altitude) = 23.00236358 meters
-------------------------------------------------------------------------------------------------------------UTM (in meters)
NAD27
686 694.72 m
WGS84
686 712.09 m
Difference
17.37 m
In my area (which is UTM Zone Number 17, Letter N), the representation of the point in UTM coordinates based on WGS84 is 17.37
meters EAST of the representation of that point in UTM NAD27.
------------------------------------------------------------------------------------------------------ --------------
page 45 in book:
re: LONGITUDE
the difference between NAD27 and WGS84 for UTM is 17.37 meters and the difference between NAD27 and WGS84 for
degrees/minutes
is 23 meters. I think that these readings are fairly close, and the two methods of calculating longitude don't yield very different results.
re: LATITUDE
the comparison between 213.54 meters (UTM comparison of NAD27 and WGS84 readings) differs so greatly from the 12.73 meters
(degrees and minutes comparison of NAD27 and WGS84) that I wonder which is more accurate for calcuating latitude.
I cannot explain this difference.
Lisa Greenbaum
GEOG 594, section 001
LAB ONE
February 13, 2007
Page 1
1. GEOID
is a nonbreaking, continuous surface at which for every point on the surface, if a tangent line or plane were to exist, the touching point
of the tangent line or plane and the geoid would be perpendicular to the force of gravity emanating from the center of the mass, and
the measurement of gravity at every point would be equal to every other. The geoid surface would represent sea level. The shape
would not have mountains or dips below sea level. It is a theoretical model of the earth.
2. REFERENCE ELIPSOID
if a two dimensional elipse were rotated about the polar axis of the earth, the result would be the elipsoid. This is a model of the
surface of the earth. The elipsoid does not create a model of uniform gravity everywhere. The center of the elipsoid represents the
center of the earth. because it is created using a 2-d elipse, the minor axis is smaller than the major axis. the satellites measure
altitude with respect to this model of the surface of the earth.
3. HAE
HAE is height above the reference elipsoid, and it is a measure of alitutde on the surface of the earth. It is not height above mean sea
level. it is height above a mathematical model that is programmed into the computers in the satellites or in the GPS receiver. The
elipsoid surface represents a gravitational surface, what is a gravitational surface?
4. An elevation mask of 5 means that a GPS receiver will look for and use data received from satellites that are at least 5 degrees in
angle above the horizon to calculate latitude and longitude and altitude and time. this is not a good value to use because of
interference from ionosphere and trophosphere and buildings or trees in the way would detract from accuracy of signal from satellite.
5. a PRN number is a pseudo random number that is used to "name" or identify each satellite that is part of the NAVSTAR
constellation. the GPS receiver uniquely identifies each satellite and the information that it receives from that satellite by this number.
6. Sat Posn and SNR screen tells signal to noise ratio -- how healthty is the signal from each satellite that the GPS receiver is reading - a higher value means a better signal and a low value means there is more noise compromising the data.
also the bearing or azimuth -- the number of degrees that the satellite is away from true north or magnetic north;
also the number of degrees that the satellite is above the horizon.
also if there is a checkmark in a box next to a PRN number, that satellite is being used by the GPS unit to compute lat, lon, alt, time.
the datum does make a difference. more recently computed datums use more accurate technology than datums computed long ago.
also, geographically the datum has to correspond to the nation or location on the earth, because datums are only accurate on a local
basis.