GIS Questions: 1. What is the difference between defining a projection and chang

Question

GIS Questions:

1. What is the difference between defining a projection and changing a projection? For a shapefile, where is this projection information stored?

2. A map of Virginia counties was developed in 1950. Each of us takes this map and places it in the same projection: Lambert conformal conic/State Plane 4502. Yet, certain features positions differ by 100 meters. Why?

3. What is the conforming property? Name something that has this property and something that does not.

4. Sometimes data can be represented a points, lines, and polygons. What type of data model is this? What is another type of data model?

5. Write a script that does the following:

reprojects the tax layer into a geographic coordinate system with 1983 datum

selects those tax parcels that are labeled with “B-1H” zoning

calculates the maximum floor area allowed in each parcel, which is 40% of the area of the parcel

writes the PIN and the floor area ratio to a csv file.

Explanation / Answer

1) Defining a projection consists of entering specific data sets when ok this doesn't know the co-ordinate system, as the data does not have a defined co-ordinate system resulting in an error message stating"unknown spatial reference". Running the define projection tool changes the metadata by describing the current projection of the data set buy creation metadata within the existing data set in the form of a .prj, .tfw, or .aux.

Changing a projection consists of changing the co-ordinate system used to another co-ordinate system for the given data set. Which can be used for a more detailed information as it can portraying a curved surface of the earth as a flat surface. The project tool has to be utilised for changing the coordinate system to change a projection and is not the same as defining a projection. Using the project tool results in changes being made to the shape file with it getting added to the data within ArcMap resulting in it being projected accordingly, when the software is opened. When the projected co-ordinate system of data is specified by using the project tool the change in the data is made in the shapefile, raster, geodatabase and all related files. the difference between defining the projection is that changes the metadata for the projection associated with the file while the project tool changes the physical co-ordinate system in the spatial file.

2) a map of Virginia counties developed in 1950, when placed on the same projection using Lambert conformal conic show certain features positions differing by hundred metres because the projection method used before 1957 was the polyconic meaning many cones. This method uses an infinite number of cones along the central meridian which results in parallel which are not concentric what is latitude representing the base of its tangential cone. The greater the distance of an area from the central Meridian the larger the distortion. local angles along the central Meridian remain accurate but with increase in distance they are distorted. There is a similar increase in distortion in the scale with an increase in distance from the central meridian.

3) The conforming property refers to the ability of a projection to preserve the shape of small areas correctly. Conformal property is visible in a projection where graticule lines intersect at right angles and the scale at any given point on the map is same in all directions. Conformal projection will also maintain every angle at each point including those between the intersections of arcs so that areas enclosed by many arcs are not distorted in size. Sachin property is visible in smaller area but not possible to replicate in projections of large regions.

4) a vector data model is used for defining discrete objects through use of data models of three types lines points and polygons. All the vector data in all three types are composed of coordinates and attributes. Polygons provide two dimensional view animal represent City boundaries or forests, with other models being used for roads, ponds and smaller objects. Another type of motor is the raster data model which uses rectangular matrices is composed of square cells for representation of the landscape. Both data models can be used individually or in combination on the basis of the required representation and type of map to be made.

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