Civil - Remote Sensing Techniques and GIS - Data Entry, Storage and Analysis
Data Entry, Storage And Analysis
1.What is Data model?
Data Models: Vector and Raster
Spatial data in GIS has two primary data formats: raster and vector. Raster uses a grid cell structure, whereas vector is more like a drawn map.
Raster and Vector Data
Vector format has points, lines, polygons that appear normal, much like a map. Raster format generalizes the scene into a grid of cells, each with a code to indicate the feature being depicted. The cell is the minimum mapping unit.
Raster has generalized reality: all of the features in the cell area are reduced to a single cell identity.
2.What is raster data?
Raster is a method for the storage, processing and display of spatial data.
Each area is divided into rows and columns, which form a regular grid structure. Each cell must be rectangular in shape, but not necessarily
Each cell within this matrix contains location co-ordinates as well as an attribute value. The origin of rows and column is at the upper left corner of the grid. Rows function as the “y”coordinate and column as”x”coordinate in a two dimensional system. A cell is defined by its location in terms of rows and columns.
3.What is vector data?
• Vector data uses two dimensional Cartesian coordinates to store the shape of spatial entity. Vector based features are treated as discrete geometric objects over the space.
• In the vector data base point is the basic building block from which all the spatial entities are constructed.
• The vector spatial entity ,the point is represented by a single x,y coordinate pair. Line and area entities are constructed by a series of points into chains and
4. What is Raster?
The raster cell’s value or code represents all of the features within the grid, it does not maintain true size, shape, or location for individual features. Even where “nothing” exists (no data), the cells must be coded.
5. What is Vector?
vectors are data elements describing position and direction. In GIS, vector is the map-like drawing of features, without the generalizing effect of a raster grid. Therefore, shape is better retained. Vector is much more spatially accurate than the raster format.
6.What is raster coding?
In the data entry process, maps can be digitized or scanned at a selected cell size and each cell assigned a code or value.
The cell size can be adjusted according to the grid structure or by ground units, also termed resolution.
There are three basic and one advanced scheme for assigning cell codes. Presence/Absence: is the most basic method and to record a feature if some of it occurs in the cell space.
7. What is Cell Center?
The cell center involves reading only the center of the cell and assigning the code accordingly. Not good for points or lines.
8.What is Dominant Area?
To assign the cell code to the feature with the largest (dominant) share of the cell. This is suitable primarily for polygons.
9.What is Percent Coverage?
A more advanced method. To separate each feature for coding into individual themes and then assign values that show its percent cover in each cell.
10.Different methods of data input?
Key board entry Digitizing
Manual digitizing Automatic digitizing
Automatic line follower Electronic data transfer
11. What is digitizing?
The most common method employed in encoding data from a paper map. Manual digitizing
Automatic digitizing Scanning
Automatic line follower
12.Write the errors in digitizing?
Scale and resolution of the source/base map.
Quality of the equipment and the software used. Incorrect registration.
A shaky hand. Line thickness. Overshoot.
Under shoot. Spike. Displacement. Polygonal knot.
13.What is scanning?
piece of hard ware for converting an analogue source of document into digital raster format (a light sensitive device).
Most commonly used method.
When raster data are there to be encoded scanning is the most appropriate option. There are three different types of scanners available in usage :-
Flat-bed scanners (a PC peripheral). Rotating drum scanners.
Large format feed scanners
14.Write the important components of scanner? A light source.
A back ground. A lens.
15.Write the practical problems in scanning?
Possibility of optical distortion associated with the usage of flat bed scanners. Automatic scanning of unwanted information.
Selection of appropriate scanning tolerance to ensure important data are encoded, and background data ignored.
The format of files produced and the input of data into G.I.S. software. The amount of editing required to produce data suitable for analysis.