Geog373(4)

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Data Collection

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Process of selecting, acquiring, and converting geographic data into a GIS

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One of the most expensive GIS activites?

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-Data collection
-Up to 85% of total cost of a GIS project

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Primary data capture

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Data captured by direct fieldwork measurement

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Primary raster data capture

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Satellite remote sensing, aerial photography

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Primary vector data capture

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Photogrammetry, land surveying, Global Positioning System (GPS)

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Secondary data capture

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Measuring data from other systems and media (digital and analog)

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Raster data collection

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Satellite imagery and pixel-based data capture

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Remote sensing

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Measuring physical, chemical and biological properties of objects remotely (without direct contact)

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Sensor functions

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Light/other EM energy is absorbed, transmitted or reflected
-Light reflected from surface and transmitted to sensor is used to create an image

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Spatial resolution

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Size of object that can be resolved by sensor

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Most usual measure of spatial resolution =

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Pixel size (typically ranging from 0.5m-1km)

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Landsat Program: Type of (raster/vector) data collection

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RASTER

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Landsat Program

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-First launched 1972
-Vital source of medium resolution imagery of Earth's surface

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Aerial Photos collected via _____

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Optical cameras

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Aerial photos taken from what platforms?

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Airplanes, helicopters, balloons, kites, tall buildings, etc.
**Form basis of the most detailed national map series in U.S.

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Photogrammetry

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Science of measuring and extracting geometry from aerial photos

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Photo interpretation

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Identifying location and properties of objects in imagery

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Not all aerial data is photographic (3)

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1. Lidar (light detection and ranging)
2. Radar
3. Hyperspectral imaging

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Digital Elevation Models (DEM)

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Raster of elevation values

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Raster calculates many useful derivatives of elevation such as _______

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-Slope/Aspect (direction of slopes)
-Visibility: What is visible from a spot?

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Land surveying is a type of (raster/vector) data collection

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VECTOR

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Land surveying

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Uses field instruments to measure locations
-Location of object is determined by angle and distance to known locations

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Global Positioning System (GPS)

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Satellite-based navigation system managed by U.S. Dept of Defense
-24 satellites used to measure locations on Earth's surface
-GPS is one of the Global Navigation Satellite SYstems

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Takes at least ___ satellites to determine a location

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4

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Geocoding

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Process of creating geometric representations for locations from descriptions of locations (ex. street addresses)

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(2) Steps to Geocoding

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1. Batch Geocoding
2. Interactive rematching

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Batch geocoding

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Attempts to match all addresses

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Interactive rematching

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Sophisticated user interface to match addresses

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Common geocoding layers:
Tabular Data

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-Street addresses
-Zip codes
-Latitude and longitude

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Common geocoding layers:
Geographic data

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-Street centerlines
-Zip code polygons

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Geocoding is a (perfect/imperfect) process

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IMPERFECT
-Difficult/impossible to match every address automatically, so manual correction needed after automated matching
-Investigate unmatched addresses
-Use online mapping to troubleshoot incorrect addresses

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Digitizing

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Process by which coordinates from hard copy maps/images are converted into digital format in GIS

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Methods of digitizing (2)

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1. Manual: human-guided coordinate capture from a map/image source
2. Scan/automated

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Hardcopy map digitizing

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Human-guided coordinate capture from a map printed on paper, plastic or other "hardcopy" material

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How does a digitizer work?

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-Wire mesh creates coordinate system of digitizer
-Digitizing puck placed over location on table--wire mesh records puck location
-Digitizer coordinates sent to computer and stored in a GIS

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Digitizing is a (perfect/imperfect) science

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IMPERFECT

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On-screen digitizing AKA _____

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Heads up digitizing

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On-screen/heads up digitizing

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Map to be digitized is an image/scanned map
-Process of manually digitizing on a computer screen with a digital image as a background

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Where to find data?

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-Agency/org
-State/county GIS data sets
-Gov agencies

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Categories of GIS data

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-Data themes: base map or domain data of environmental, socio-economic, etc.
-Data components: spatial data (raster, vector), attribute data
-Data ownership: private, public data

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Major available data

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-Topography
-Elevation
-Hydrology
-Transportation
-Satellite images
-Aerial photos
-Land cover
-Census data

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GNIS

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Geographic Names Information System

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Data quality

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Describes how faithfully digital representation in GIS reflections true shape, location and characteristics of real world phenomena

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Measures of data quality

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-Accuracy
-Precision
-Error

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Accuracy

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Measure of "how close" data are to TRUE values
-Positional accuracy
-Attribute accuracy

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True values can NEVER be exactly determined or known because...

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-Limitations of instruments
-Human inability to perform perfect observations
-Higher order positions accepted as true values

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Precision

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Measure of "how exactly" data are measured/recorded
-Refers both both: number of sigfigs AND measurement's repeatability

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Error

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Measure of how DIFFERENT data are to true values

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Data quality often expressed in terms of _____ than accuracy/precision

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ERROR

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(2) Sources of errors in geospatial data

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1. Inherent errors
2. Operational errors

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Inherent errors

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-Imperfection of conception, measurements, representation of real world
-Naturally occurring

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Operational errors

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-Occur as result of imperfection of instruments and methods for geospatial data collection, management and application

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Uncertainty

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Lack of confidence in use of data that is due to incomplete knowledge of data
-ALWAYS EXISTS

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Positional accuracy

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How far a feature is from its true location

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Attribute accuracy

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Difference between attributes are from true values

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Logical consistency

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Consistency of data model to the real world--how well data can adequately represent features in real world

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Completeness

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How well data captures all characteristics it is intended to represent

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Semantic accuracy

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Measure of how correctly spatial objects are labeled/named in a dataset

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RMSE gives overall ________ accuracy

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POSITIONAL

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Rule of thumb for positional accuracy:
Positional accuracy of features on paper map is roughly ____ on map

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0.5mm
**Multiply by scale of map to give corresponding distance on ground**

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Confusion Matrix for ______ attribute accuracy

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Nominal
-Compares recorded attributes with attributes obtained by a more accurate process

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Error Propagation

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One error leads to others, spreads through sequence of operations and/or data

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MAUP

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Modifiable Areal Unit Problem
-Challenge that occurs during spatial analysis of aggregated data
-Different aggregation schemes of same data result in different analysis results