Flashcards in Geodesy Surveying Deck (47):
What is geography?
Geography is a field of science dedicated to the study of the lands, the features, the inhabitants, and the phenomena of the Earth. A literal translation would be "to describe or write about the Earth".
Who was the first person to use the word geography?
What are the four historical traditions in geographical research?
1. Spatial analysis of natural and human phenomena (geography as the study of distribution – the GIS thinking questions we just did),
2. Area studies (places and regions – GEOG 1014 – World Regions),
3. Study of the human-land relationship
4. Research in the Earth sciences.
Why does geography matter to GIS? What specifically?
Because geography is the basis of GIS. Specifically location is the basis.
Geographers study and model space and make some very strong contributions to geospatial science. What are these contributions?
They observe patterns, and then try to explain them using various tools of which GIS is a major one (along with statistics, mathematics, remote sensing, survey, qualitative analysis, and quantitative analysis).
Geographers work with other disciplines to bring a spatial perspective to many types of problems (engineering like wireless telecommunications, navigation, agriculture, medicine and disease, etc.)
What is geodesy?
The science of the size and shape of the Earth.
Questions on the size and shape of the earth go way back and three dominant theories have been followed:
What is the flat earth theory?
Today this idea sounds silly and the “flat Earth society” does not strike anyone as being a serious group, but at the time when people could observe only very local phenomena, it was perfectly reasonable.
What is the distance to the horizon of the earth? In other words, what simple formula tells us the "flat" earth portion that we can experience in open space? What does "d" signify? What does "h" signify?
d = 1.22*(squareroot of h)
d = miles
h = feet
What were Aristotle's three theory's for a spherical Earth?
1. Ships appear to sink at the “edge” not plummet, and they do return
2. Stars hold different positions in the night sky in different places. Polaris (North Star) appears higher above the northern horizon as you go north.
3. Lunar eclipses show a curved edge to the Earth.
When was earth's circumference first estimated? Where? By Whom? How?
The Earth’s circumference was first estimated in Egypt by Eratosthenes in about 220BC. Taking Aristotle’s sphere, he calculated the distance for a portion of a circle around Earth and then multiplied to get the total.
One day per year a well in Syene was completely illuminated - the sun was directly overhead for a short time
This did not occur in Alexandria at the same time so he computed the angle of the shadow cast.
What was the ellipsoidal earth theory?
By 1670, mathematician Sir Isaac Newton developed a theory that the Earth could not be a perfect sphere because of the forces of rotation. He theorized that the earth was actually bulging outward at the equator a bit. His ideas led to an expedition measuring the exact length of a degree of latitude in the far north versus the equatorial regions to find that the degree was longer at the pole than at the equator.
1. What did Newton propose the amount in the difference of the axes was?
2. What is flattening estimated as today?
What is the real shape of the earth?
A geoidal shape.
What does the geoid assume (7 assumptions)?
1. Earth has only a water covering with no land to disturb the surface
2. The water has no tides
3. The water has no currents
4. The shape is the distance of the hypothetical water surface from the center of Earth at every location.
5. Measured to a grid with gravity sensing satellites.
6. Based on unequal distribution of mass – rock density and crust thickness – more dense, more gravitational pull – “lower” Geoid
7. Used to approximate “Mean Sea Level” around the world
What is the best fit for selecting ellipsoids?
Once we agree on the size and shape of Earth (not that we all do), we can develop measurement systems. What are the two key systems?
1. Geographic Coordinate System
2. Cartesian Coordinate Systems
What is the geographic coordinate system?
latitude/ longitude angular measurements
What is the cartesian coordinate system?
X, Y (easting, northing) distance measurement systems
What is the earth grid system based on? What is this called?
Angles of arc around the surface. This is called the Graticule.
What is the graticule made of?
Made of lines of:
1. latitude - measurements made north and south of equator
2. longitude: measurements east and west of the prime meridian - established in 1884.
What are parallels?
Lines that measure position north/ south
Parallels are (longer/shorter) away from the equator?
Parallels are (Parallel/perpendicular) to one another and are (almost/ not) equally spaced. They range from ________. (N: (+/-), S: (+/-)
What are meridians?
Lines that measure position east/ west.
Meridians are (equal/ not equal) in length. They are spaced widest at the equator with __________ to the points of the poles. They range fro__________. (E: (+/-), S: (+/-)
What are the advantages of the geographic coordinate system?
1. Single System that “fits” the entire globe without any interruptions
2. Most accurate for positions for large GIS study areas
What are the disadvantages of the geographic coordinate system?
1. Measures ANGLES not distances, not easy to derive lengths, areas, or volumes for GIS measurements
2. Distance of one degree is not constant at all locations
Geographic Coordinates are specified in two manners, one used mainly on maps, the other by computers. What is it?
1. Degrees: Minutes: Seconds (D:M:S) is used on maps and reflects the geometric system of angular measurements. Uses N, S, E, W designators
What are decimal degrees?
Are commonly used in computing as they are easier to deal with being built on 1/10 parts instead of 1/60 parts.
These are easy to convert.
What are Cartesian systems based on?
Two right angled axes on a plane.
For Cartesian systems, what is the origin? What are coordinates?
1. “Origin” is the starting point. This point is usually an arbitrary location.
2. Coordinates are the distance of a point from the origin along the axes in 2 dimensions
What are advantages to the Cartesian system?
1. Measure location in units of distance rather than in angles
2. Distance, area and volume measurements are very straightforward and simple in GIS. They are also accurate as the input allows.
What are disadvantages of the Cartesian system?
1. Do not fit the sphere or ellipsoid models of earth. Cartesian grids are only possible on a plane (“Flat earth”) view. This causes locational error and requires careful choice of map projection to accommodate desired Earth qualities.
2. Systems have no “natural” starting place for the origin so they are arbitrary, generally SW of the region to be mapped.
1 degree of latitude = ___ miles.
1 degree of longitude = ___*_____
When converting angular measurements to distance, the measurements are based on______.
These are based on a perfect spheroid Earth, which is not true, so they yield approximate, though quite close, answers. When GIS uses these formulae, your results are a little off.
GEOID distances have been measured and are (more/less) accurate than our formula?
What is the key for all Geodectic work?
The key to all of the Geodetic work was to establish a basis for measurements on the surface.
What can we do with a coordinating system?
With a coordinate system in place we can go out and start measuring locations on Earth.
Since surveyors always measure relative to previously established control points, what do we need to to?
Establish a few control points on each continent from which to start
From a known point, what can survey traverses be based on?
Mathematics - usually to a final known point to assure accuracy
Triangulation allows us to only see across _____. This means results are derived (more/ less) quickly
1. Short distances (flat earth)
What do Polar Coordinates consist of?
Consist of a direction (azimuth) and horizontal distance from a known location. (slide 5 in Geodesy Survey for pictures)
How does we compute the location of triangulation points?
A triangle’s angles always sum to 180⁰
Law of Sines:
On any triangle, all sides divided by the sin(opposite angle) are equal
a/sin(A) = b/sin(B) = c/sin(C)
What does a datum define?
The starting position of a spheroid/ ellipsoid relative to the center of the earth.
What does a datum provide?
A frame of reference for measuring locations on the surface of the earth by defining the origin and orientation of latitude and longitude lines