Lecture 6 (GIS Data Collection) Flashcards Preview

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Flashcards in Lecture 6 (GIS Data Collection) Deck (27)
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1

Data Collection

Adding geographic data into a GIS database.
- One of most expensive and time consuming GIS activities
-Many diverse sources
- Two broad types of collection
-> Data capture
-- Two broad capture techniques (direct entry)
• Primary (direct measurement)
• Secondary (indirect derivation)
-> Data transfer
– importing the existing data from other sources into a GIS database
DATA MAINTENANCE IS MORE IMPORTANT AND COMPLEX THAN DATA COLLECTION

2

Data Collection Techniques

- Primary data sources are captured specifically by direct measurement.
- Secondary sources are reused or obtained from other systems.
SEE SLIDE

3

Primary Data Sources

Digital and analog format
Raster - Digital remote sensing images/ digital aerial photographs
Vector - GPS measurements/ Survey measurements

4

Secondary

Digital and analog format
Raster - scanned maps/ photographs/ DEMs from maps
Vector - USGS Topographic maps/ Toponymy (place-name) data sets from atlases

5

Primary way

Measure terrain directly/ Digital imagery collected with LiDar

6

Data Collection - Digitalizing

- Analog data must be digitized before adding to a GIS database.
- Analog-to-digital transformation
-> Scanning paper maps or photographs
-> Optical character recognition (OCR)
-> Text describing geographic object properties
-> Vectorization of selected features

SEE SLIDE

7

Data Collection Project Workflow

1) Planning (establish users' requirements, gathering all resources), 2) Preparation (obtain existing data, hardware/ software), 3) Digitizing/ Transfer (Require most efforts, Survey data entry, scanning, photogrammetry), 4) Editing/ Improvement (Validate data, Correct errors, improve quality), 5) Evaluation (Identify if the project succeeded, or failed)

8

Primary Data Collection

Raster
- Remote Sensing (Aerial Photography, Satellite Imagery)
Vector
- Ground Survey
- GPS (Global Positioning System)
- LiDAR (Light Detection and Ranging)

9

Raster Primary Data Capture

- Remote sensing – the most popular form
-> The measurement of physical, chemical, and
biological properties of the objects without direct
contact.
-> Information derived from the measurement of the amount of electromagnetic radiation reflected,
emitted, or scattered from objects.
-> Passive sensors (limitation: cloud effect)
• Rely on reflected solar radiation or emitted terrestrial radiation (by sun light)
-> Active sensors – synthetic aperture radar (SAR)
• Generate their own source of electromagnetic radiation

10

Raster Primary Data Capture

Remote Sensing
- Satellite imagery
-> Earth-orbiting satellites
– collect information from parts of Earth surface at regular time intervals.
- Aerial photography
-> Fixed-wing aircraft
-> Smaller area in great detail
-> Suitable for detailed surveying and mapping projects

11

Raster Primary Data Capture - Remote Sensing (Four Key Aspects of Resolution)

- Spatial resolution – pixel size: The size of object to be resolved
- Spectral resolution
-> The parts of electromagnetic spectrum to be measured
-> Different objects emit and reflect different types and amounts of radiation (e.g., Landsat 8, 0.435 nm ~ 2.297 nm; 11 bands)
-> Single band (value range: 8 bit [0-255]) or multiband/multispectral
- Radiometric resolution- How finely the radiometric resolution a system can represent or distinguish differences of intensity, and is usually expressed as a number of bits (8 bit [0-255]).
- Temporal resolution – repeat cycle
-> Describe the frequency of images to be collected for the same area

ALL SENSORS NEED TO TRADE OFF BETWEEN THESE RESOLUTIONS, B/C OF STORAGE PROCESSING AND BANDWIDTH CONSIDERATION

12

GeoEye 1 (2008) and 2 (not currently scheduled to launch)

Geoeye2 : .34m spatial, spectral, radiometric, temporal
4 multispectral bands
SEE SLIDE

The resolution of the satellite image is 0.41 meter in panchromatic image and 1.65 m in multispectral imagery (Repeat cycle is between 2.1-8.3 days).

13

Worldview 2 - Satellite

8 multispectral bands 2.0m
Panchromatic (B&W): .5m
The resolution of the satellite image is .5 m

14

Vector Primary Data Capture

- Ground Survey - Shifting from measurement-based to coordinate-based
- GPS (Global Positioning System) - Receiving the orbit satellite signals to calculate the location through trilateral
- LiDAR - Light Detection and Ranging - Using scanned laser range-finder to produce accurate topographic surveys with great detail.

15

Vector Primary Data Capture

Surveying
-> Principle: 3D locations of objects determined by angle and distance measurements from known locations (benchmark point)
-> All points obtained from survey measurements, and their locations are relative to other points.
-> Time-consuming and expensive activity
-> Uses expensive field equipments and crews
-> Most accurate method for large scale, small areas
-> Provide the georeference points for other fine-scale images

Ex. Leica Total Station

16

Vector Primary Data Capture

GPS – Global Positioning System
-> Consist of a system of 24 satellites, orbiting the Earth every 12 hrs, at the elevation of 20,200km
--> Transmitting radio pulses at precisely timed-interval
-> Four satellites to locate a three dimension (x,y,z)
-> Differential GPS (DGPS) used to improve accuracy
------> Combines GPS signals from satellite with correction signals via radio/telephone from base station

Pen Portable PC and GPS - Originally funded by the US Dept. of Defense
• Navigation
• Mapping
• Surveying
• Other applications
needed precise positions
EX. Kinematic GPS Surveying

17

Vector Primary Data Capture

LiDAR (Light Detection And Ranging)
-> Active sensor
-> Using lasers to measure distances to reflective surfaces
-> Point cloud: a massive collection of independent points (x, y, z)
-> Collect extremely large quantities of very detailed information
• 200,000 points per second
• Accuracy around 10 cm

Ex. Multiple Pulses in Air (MPiA) doubles pulse rate
Alliance for Integrated Spatial Technologies (SLIDES)

18

Secondary Geographic Data Capture

- Data collected from other purposes can
be converted for use in GIS projects
- Raster conversion
-> Scanning
- Vector conversion
-> Digitizing/Vectorization
-> Photogrammetry
-> COGO – coordinate geometry

19

Raster Conversion

-> Scanning – convert hard-copy analog media into digital images by scanners
• Scanner – scan the line across the map to record the amount of light reflected from data sources
-> Sources:
• Maps, Aerial photographs, Documents, etc

20

Digitizing/ Vectorization

-> Process of converting raster data into vector data
-> Collection of vector objects from maps, photographs, image, etc.
• Primary sources
• Secondary raster data

21

Georeference/ Georegistration

-> Convert image coordinates into database coordinates by geometric transformation process/algorithms
• Photo image
• Coordinate database
• Three reference/control points

22

Image Rectification Using Ground Control Points (GCPs)

1. Identify GCPs on image and reference
2. Develop transformation equations
3. Resample to determine digital number (DNs) for
corrected image pixels
1st Order Polynomial
2nd Order Polynomial

See SLIDE

23

Resampling

- Used to determine the pixel DN values to fill in the output matrix by sampling from original (distorted) image
- Nearest Neighbor
-> Uses closest pixel value
-> Discreet data
• Classes, land use
- Bilinear Interpolation
-> Uses 4 closest neighbors
-> Continuous data, localized
• Elevation data
- Cubic Convolution
-> Uses 16 closest neighbors
-> Continuous data, smoothest (IMAGES)

SEE SLIDEs (EXAMPLES)

24

Vector Secondary Data Capture - Digitizing/ Vectorization

Manual digitizing – vertices defining point, line, polygon
-> Point – screen cursor to record the location by clicking button
-> Stream-mode – partially automated collecting vertices ( .25 sec, .02in)

EX. Heads-Up Digitizing and Vectorization

25

Photogrammetry

-> the science and technology of capturing measurements from images
-> 2D measurement – point, line, area
-> 2.5D or 3D measurement – height, elevation
• Stereopair of photographs
– 60% overlap along each flight line
– 30% overlap between flight lines
-> View stereo/3D model
• Split screen with stereoscope
• Special glasses to observe
red/green display or polarized light

SEE SLide (Ex. Stereo Pair, 2.5D v 3D)

26

Coordinate Geography (COGO)

- Capturing and representing geographic data
- Same principles as the survey
-> Bearings, distances, and angles to define each part of an object
- The only legally acceptable definition of land records and property parcels in U.S.
-> Very precise measurements
-> Highly qualified individual objects

COGO stands for coordinate geometry, it is
a vector data structure and method of data entry.

SEE SLIDEs

27

Data Transfer

- Buy vs. build is an important question
- Many widely distributed sources of GI
- Finding existing map data
-> Map library – network search, or media such as CD-ROM and disk
-> Internet search – World Wide Web
-> Federal, state, and local agencies (Geospatial one-stop, NSDI - Data clearinghouse, USGS - National Map, EROS, NOAA, Census Bureau, EPA, USDA)
-> GIS vendors package data with commercial
products (geocommunity, geoplace)

Portland Metro's Regional Land Information System (RLIS)