Rectifying process is the process of shifting pixel locations to remove distortions, and very often the rectification includes georeferencing, because one can both shift the pixels to remove distortion and assign coordinates to those pixels at the same time.
2. What is Ground Control Points (GCP) in rectifying process?
Ground Control Point (GCP) in rectifying process is a specific pixel on an image or location on the surface of the Earth.
3. What is map projection?
Map projection is the attempt to portray the surface of the Earth or a portion of the Earth on a flat surface. Map projection causes distortions of distance, direction, scale and area. In some projection, it minimises distortions in some of the properties mentioned above, by expending maximum errors in other properties. Different map projections will result in different spatial relationships between regions, which also mean that different projections cause different types of distortions.
4. What is geodetic datum?
Geodetic datum is defined as the size and shape of the Earth, the origin and orientation of the coordinate systems (latitude and longitude lines) used to map the Earth. Or, to further describe geodetic datum, it provides a frame of reference for measuring the location on the surface of the Earth. Datums have evolved from a spherical Earth to ellipsoidal models derived from years of satellites measurements. One of the modern geodetic datums is the flat-earth models, which is used for plane surveying, while the complex model is used for international applications. Some characteristics which are completely described by the complex model are the size, shape, orientation, gravity field, and also the angular velocity of the Earth.
**For your information, the first estimation of Earth’s size was made by Aristotle.
5. Can two maps of different map projection can be overlay onto each other? Provide the reason/s.
Two maps of different projection are best not to be overlay onto each other. This is because the Earth’s surface cannot be flattened without distorting geometrical properties. Geographical properties include area, shape, distance and direction. These spatial properties can be preserved individually and in certain combinations on map projections, however, these four basic properties cannot be held true simultaneously. Besides, different projections will cause different distortions, and a suitable projection will be chosen to suit the mapping situation. Therefore, if maps of different projections are to be overlay onto each other, many distortions and errors will occur, which directly decrease the accuracy of the maps to be produced.
6. How many GCP that you had utilised in georeferencing the aerial photos? Provide the table of the GCPs in your report.
I had utilised 7 Ground Control Points (GCP) in georeferencing the aerial photos.
I had utilised 7 Ground Control Points (GCP) in georeferencing the aerial photos.
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| GCPs for File 124 |
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| GCPs for File 125 |
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| GCPs for 126 |
7. What are the criteria of a good GCP?
The criteria of a good Ground Control Points (GCP) is when the points chosen are:
i) uniformly distributed
ii) respect the terrain variations in the scene (selecting of points at both highest and lowest elevations)
iii) clearly identifiable points in the image
8. Provide three examples of good features for GCP.
Examples of good features for GCPs:
i) cultural feature
- roads and railroads intersections
- river bridges
- large low buildings (industrial buildings)
- airports
ii) line feature
- well defined edges
- two line features forming intersection with an angle larger than 60 degrees
iii) natural features
- not preferred due to their irregular shapes
- used in areas lacking of suitable cultural features
- examples: forest boundaries, forest paths, forest clearings, river confluence
9. What is the min value of RMSE for each point that was used that you manage to achieve?
The minimum value of Root Mean Square Error (RMSE) for each points that was used that I managed to achieve is 0.03.
10. Why do you think it is difficult to achieve the RMSE of less than 1 in this aerial exercise?
I think that it is very difficult to achieve the RMSE of less than 1 in this aerial exercise due to human errors, such as inaccuracy during selection of GCPs and lack of observations during selection of GCPs.
11. Why it is important to achieve RMSE of less than 1?
RMSE is the square root of the mean or average of the square of all of the error. In Geographic Information System (GIS) dictionary, the RMSE is a measure of the difference between locations that are known and locations that have been interpolated or digitised. It is derived by squaring the differences between known and unknown points, adding those together, dividing that by the number of test points, and then taking the square root of that result. Therefore, it is important to achieve RMSE of less than 1, for it indicates the error occurs during the selections of GCPs on different maps. The smaller is the value of RMSE, the smaller is the error, the higher is the accuracy of the map, provided that the value is less than 1.
12. What is/are the difference/s between polygon, polyline and point in feature type?
Point feature is a geometric element defined by a pair of x-coordinate and y-coordinate. Polyline is a shape defined by one or more paths. Paths here referred to a series of connected segments. If a polyline has more than one part, or known as a multipart polyline, the paths may either branch out or discontinuous. Meanwhile, a polygon is a closed shape defined by a connected sequence of x and y coordinate pairs, where the first and last coordinate pair are the same and all other pairs are unique on a map, while in ArcGIS software, a shape is defined by one or more rings, where a ring is a path that starts and ends at the same point. If more than one ring exist in a polygon, the rings may be separated from one another, or might be inside one another, but they will never overlap one another. Difference between point, polyline and polygon is point does not connect each other, it is just a point of x and y-coordinates, while polyline is an open series of points, from the first till the last point, and lastly, polygon is always closed. There is a line from the first point, which will also be the last point, which makes it a closed shape. A polygon can either be filled by default, or can not be filled. However, a point and polyline can never be filled.
12. What is/are the difference/s between polygon, polyline and point in feature type?
Point feature is a geometric element defined by a pair of x-coordinate and y-coordinate. Polyline is a shape defined by one or more paths. Paths here referred to a series of connected segments. If a polyline has more than one part, or known as a multipart polyline, the paths may either branch out or discontinuous. Meanwhile, a polygon is a closed shape defined by a connected sequence of x and y coordinate pairs, where the first and last coordinate pair are the same and all other pairs are unique on a map, while in ArcGIS software, a shape is defined by one or more rings, where a ring is a path that starts and ends at the same point. If more than one ring exist in a polygon, the rings may be separated from one another, or might be inside one another, but they will never overlap one another. Difference between point, polyline and polygon is point does not connect each other, it is just a point of x and y-coordinates, while polyline is an open series of points, from the first till the last point, and lastly, polygon is always closed. There is a line from the first point, which will also be the last point, which makes it a closed shape. A polygon can either be filled by default, or can not be filled. However, a point and polyline can never be filled.
