Terms and definitions * keep definitions of processes and terms that may be useful for tests, assignments

Transcription

1 Lecture 1 Core of GIS Thematic layers Terms and definitions * keep definitions of processes and terms that may be useful for tests, assignments Lecture 2 What is GIS? Info: value added data Data to solve a problem, data that serves a purpose Information systems: transform data into information Aka database Give tools we need to manipulate to attach meaning to data Locational info may change interpretation of data, and is different Anatomy of GIS Input > pre-processing/data management > analysis > output Geographic data Organised in layers Each layer contains data about one type of thing Maps made of multiple layers Data models: Raster: image/surface data, grid of cells where every pixel has a value (one per) Continuous source of data Vector: formed of nodes (connected by arcs) 3 types - points, lines, polygons Each item is a feature and data only exists where there are features Geographic data is multidimensional X,y - spatial dimension Z - attribute dimension Attribute table Sources of spatial data Digitizing Remote sensing Collected by satellite/aircraft Usually raster Fieldwork, GPS Pre-processing

2 Screen data Organise for retrieval and editing Correct distortions - geometric and atmospheric Rescale Reproject Database operations Query by attributes Relational operation Link attribute table to other tabular data by shared unique key field Can only analyse existing features and attributes belonging to same features Geographic operations 1. Measurement 2. Query by location 3. Overlay operation -using shared location information. 4. Dissolve operations - merges based on certain common characteristics 5. Local operations 6. Zonal operations 7. Neighbourhood operation 8. Topographic variables Can be estimated from digital elevation model DEM 9. Network analysis operations

3 DATA FORMATS AND TYPES Overview 1. Data models 2. Types of variables 3. Data storage metadata: data about data Data models Raster - grid of pixel values, each has a value Vector - irregular, series of points possibly connected into lines, polygons Raster formats 3 types Each layer in multidimensional raster is a band Represented as long string of pixel values Differ in interleave: way the bands are shuffled together BSQ: band sequential interleave Takes every pixel value in long string of numbers in one band, and tacks on the next values for next band and so on - SEQUENTIAL Generic raster format used by Terrset BIP - band interleaved by pixels Shuffles pixel values around Listing all values for one pixels then moves to next and lists all values and so on Good for spectral processing - similar to local analysis (performing analysis using all values for that one pixel) BIL - band interleave by line Hybrid between previous 2 - partially mixed up Lists all values for band for first line, continues until completed all bands and all lines Reasonable efficiency Spatial refencing in rasters Uses one pixel s size and co-ordinates to calculate other pixel locations in image Different points of reference in different GIS programs Pixel dimensions may be different Vector representation Header info For file itself, and features within file What kind of vector file? Point, line polygon? Do features have subparts within? Bounding box - min and max spatial co-ords covered by a vector file/feature within vector file????? Increase efficiency of computation File contents Arc shapefiles have other help files - stuff is saved externally

4 Ordering Ordering influences shape and topology of resulting feature Topology: Spatial associations of a feature Ordering of points influences through e.g. sides of line (left, right/right, left) Polygons follow right hand rule: side to right of line is inside of the polygon Raster vs vector VECTOR: Small files unless Linked to attribute tables Might give additional info to features Topological info Appropriate for discrete locations, boundaries - works well for manmade objects Hard to do stuff like finding area of a thing Raster: Larger files Not explicitly linked to database Matrix representation Good to rep. Continuously varying variables Picking one Depends on what kind of analysis you want Types of variables Either quantitative or qualitative Boolean Qualitative - contains logical information (TRUE/FALSE) or 1/0 Good for suitability analysis along individual variables or combination of conditions (like in Ex. 3) Good for image processing and remote sensing Turning on or off pixels Can increase precision Categorical Qualitative can t do numerical operations Although have variables they are Dummy variables and don t mean anything Ordinal Relationships between class values Logical progression or ranked list Not appropriate to perform math operation Continuous Contains numerical values that you can perform math operations on Data storage Data types May determine what s appropriate or not for project Binary Circuits exist in two states: ON or OFF

5 Computers use binary numbers that have 2 possible values (0 or 1) which corresponds to a circuit Each digit in binary number known as a bit Stick digits together to create a complete binary number Number of digits will influence total values possible to represent 1 bit can have either 2 possible values Multiples of 8 8 bits together is known as a byte 256 possible values Files in GIS stored in binary as more efficient Text files ASCII format used Each character stored with separate bytes worth of data Codifies which numbers within numbers corresponds to which character which is then used in text file Numeric data types 3 numeric data types available in terrset: Byte Short integer Floating point (real) map displays When data converted to make display, computer only deals with byte data types Conversion of data types to byte range (0, 255) Applies display values to a given colour palette to produce a map display Be aware of data type Range of values Required precision Problems can arise for division File sizes Cannot use attribute table if its larger than 2gb File formats