Representation and Input This is lecture 3. Digital vs Analog The only really useful GIS information is digital.

Representation and Input

This is lecture 3

Digital vs Analog

• The only really useful GIS information is digital.

Analog

• In the case of analog or traditional maps, information is fixed.

• Analog maps literally use analogies (lines for roads, blocks for houses, blobs for towns) to represent the earth.

Digital 1

• A digital map displays information on the screen but the properties such as scale and projection are not fixed.

Digital 2

• In its digital form, the road is represented by a series of coordinates.

Digital 3

• In contrast to the analog model, it is the geographic data that are the basis of the GIS representation, and not the maps displayed on the screen.

Getting information into the computer

• Need to geocode spatial data.

• Geocoding is the conversion of spatial information into digital form.

• Geocoding involves capturing the map, and sometimes also capturing the attributes.

• The process also involves formatting data so that it can be used by the computer.

Three traditional methods of geocoding

• Digitizing

• Scanning

• Field data collection

• New methods include: WWW download and GPS

Important things to remember about data

• data input is a major bottleneck in application of GIS technology

• costs of input can be a major (or the major) expense associated with a project

• data input is labor intensive, tedious, error-prone

More about data

• sharing of digital data is one way around the input bottleneck

• more and more spatial data is becoming available in digital form

• It is also important to remember that data input to a GIS involves encoding both the locational and attribute data

Location data

• the locational data are encoded as coordinates on a particular cartesian coordinate system

• source maps or downloaded data may have different projections, scales

• several stages of data transformation may be needed to bring all data to a common coordinate system

Digitizing 1

• Historically the first method of inputting data was digitizing.

Main steps in digitizing

• The map, photo, or other document is placed on the flat surface of the digitizing tablet

• the position of an indicator as it is moved over the surface of the digitizing tablet is detected by the computer and interpreted as pairs of x,y coordinates

• the indicator may be a pen-like stylus or a mouse.

Steps 2

• three or more control points ("reference points", "tics", etc.) are digitized for each map sheet

• these should be easily identified points

Digitizing modes

• Digitizing the map contents can be done in two different modes:

• In point mode, the operator identifies the points to be captured explicitly by pressing a button

• In stream mode points are captured at set time intervals (typically 10 per second) or on movement of the cursor by a fixed amount

• Most digitizing is done in point mode.

Problems with digitizing

• most maps were not drafted for the purpose of digitizing

• paper maps are unstable

Problems 2

• errors occur on these maps, and these errors are entered into the GIS database as well

• the level of error in the GIS database is directly related to the error level of the source maps

Problems 3

• maps are meant to display information, and do not always accurately record locational information

• discrepancies across map sheet boundaries can cause discrepancies in the total GIS database

• user error causes overshoots, undershoots (gaps) and spikes at intersection of lines

Digitizing errors

• Error 1 – sliver error (caused by space between separately closed polygons);

• Errors 2 & 3 – line closing errors. Some systems allow you to set a certain tolerance and if points fall within a tolerance zone of each other, they are assumed to be at the same location.Errors 4, 5, & 6 – due to missing segments, wrongly labelled segments of twice-digitized segments

Errors 2

• Errors 7 & 8 – (weird polygons) due to careless digitizing or poor quality source document

• Error 9 – occurs when two maps have been separately digitized and then need to be combined. This is a process called edge-matching.

• User fatigue and boredom. • Luckily, many digitizers detect some errors

automatically and correct them

Scanning as a form of data input

• Scanners are a faster, less labour intensive method of data input.

• There are various types of scanners including:- drum scanners (very high quality) and - flat bed scanners.

Requirements for scanning

• In order to scan a document:

• documents must be clean

• lines should be at least 0.1 mm wide

• complex line work provides greater chance of error in scanning

Problems with scanning

• text may be accidently scanned as line features. i.e. text streamed along a river becomes a wider part of the river.

• contour lines cannot be broken with text • automatic feature recognition is not easy (two contour

lines vs. road symbols) • special symbols (e.g. marsh symbols) must be recognized

and dealt with • if good source documents are available, scanning can be an

efficient time saving mode of data input • Unfortunately, most maps do not lend themselves to

efficient scanning.

Vector maps are rasterized in order to scan. Then the polygons are re-created in vector.

Field data collection

• Traditionally, geographers have conducted their own data collection.

• This usually involves collecting samples or recording events at know geographic positions.

• Traditionally, the “known geographic location” specification was restricting.

• The introduction of GPS has eliminated this constraint.

Buying or downloading data

• First choice of many small users.

• Need to convert between formats (FME)

• Sources include:

• USGS (United States Geological Survey) http://www.usgs.gov/

• Census data. www.census.gov/

• GeoWeb. www.ggrweb.com

Global positioning systems (GPS)

• GPS is becoming one of the most important technology that intersects with GIS

• a new tool for determining accurate positions on the surface of the earth

• computes positions from signals received from a series of satellites (NAVSTAR)

• depends on precise information about the orbits of the satellites

GPS use and accuracy

• particularly valuable for establishing accurate positional control in remote areas

• accuracy will continue to improve as more satellites are placed in orbit and experts fine tune the software and hardware

About Navstar satellites

Name: NAVSTAR • Manufacturer: Rockwell International • Altitude: 10,900 nautical miles • Weight: 1900 lbs (in orbit)• Size:17 ft with solar panels extended • Orbital Period: 12 hours • Orbital Plane: 55 degrees to equitorial

plane • Planned Lifespan: 7.5 years

How GPS works

• GPS uses satellites (at least 3) to record positions on the earth’s surface. The calculations are done through a process of triangulation.

• Radio signals are sent by 24 satellites to GPS receivers.

• The system works by measuring how long it takes a radio signal to get from the satellite to the earth.

• There is error in the calculations and ground stations on earth are used to correct some of that error.

• More precise locational data is possible since May, 2000

Source: Longley, P.A., M.F. Goodchild, D. J. Maguire, and D.W. Rhind, eds. 2001. Geographical Information Systems and Science. New York: John Wiley & Sons, Inc, p. 212.

Mobile GPS

• Portability of GPS has led to proliferation of location-based services.

• Allows users to update data in the field in real time.

• Some systems allow two-way data flow.• Allows tracking (digital angels).• Studies of movement (time-space geography)

Differential GPS

• The quest for greater and greater accuracy has spawned an assortment of variations on basic GPS technology.

• One technique, called "Differential GPS," involves the use of two ground-based receivers.

• One monitors variations in the GPS signal and communicates those variations to the other receiver.

• The second receiver can then correct its calculations for better accuracy.

Carrier-phase GPS

• Another technique called "Carrier-phase GPS" takes advantage of the GPS signal's carrier signal to improve accuracy.

• The carrier frequency is much higher than the GPS signal.

Augmented GPS

• The aviation industry is developing a type of GPS called "Augmented GPS" which involves the use of a geostationary satellite as a relay station for the transmission of differential corrections and GPS satellite status information.

Attributes of spatial data

• Location and shape files have attributes which must also be entered and then linked up to the shape or point.

• Each shape will be associated with a list of variables (Var1…Varn).

• They might represent such things as area in square miles, population, highest altitude within the polygon etc.

Source: Longley, P.A., M.F. Goodchild, D. J. Maguire, and D.W. Rhind, eds. 2001. Geographical Information Systems and Science. New York: John Wiley & Sons, Inc, p. 212.

Characteristics of attribute data

• attributes usually obtained through a combination of field collection and interpretation

• categories may be subjective attributes such as these may not be easy to check in the field

• for social data, a major source of inaccuracy is undercounting

Spatial data infrastructure (base maps)

• Spatial data forms the infrastructure for businesses and government agencies to plan.

• Infrastructure data is based on framework data

• Street centerline databases form the backbone of GIS and computer mapping applications.

Street network data

• Street centerline spatial data is encoded with attributes such as street names, address ranges, Postal Codes, and census boundaries.

• These are then combined with non-street features such as bodies of water, railroads or landmarks to create a spatial data infrastructure that is used in a variety of business and government applications.

• Street data is important because it expresses the fundamental relationships between street addresses (the most common spatial reference point for most of us) and coordinates or other locational links.

The role of the Census

• In Canada, the Canada Census street network file is compiled with each new enumeration.

Maintaining base data

• To be useful, spatial data must be constantly maintained.

• New streets are added every day, and Postal Codes, and letter carrier routes change frequently.

• Successful private database companies, such as DMTI have concentrated on street and address currentness and completeness.

Data policies in Canada vs US

• US has different spatial data policies than other developed nations.

• Spatial data infrastructure is growing more slowly in Canada, UK, New Zealand and Australia