Projections & GIS Data Collection: An Overview

Projections Primary data capture Secondary data capture Data transfer Capturing attribute data Managing a data capture project

Geodesy Basics for Geospatial Data

Geodesy: The study of the Earth’s size and shape. or, more formally: ”A branch of applied mathematics which

determines by observation and measurement the exact positions of points and the figures and areas of large portions of Earth's surface, the shape and size of the Earth, and the variations of terrestrial gravity.”

Ellipsoid (Spheroid)

major axis half axis: semi-major axis (a)

minor axis half axis: semi-minor axis (b)

The History of Ellipsoids

Because the Earth is not shaped precisely as an ellipsoid, initially each country felt free to adopt its own as the most accurate approximation to its own part of the Earth

Today an international standard has been adopted known as WGS 84 Its US implementation is the North American Datum of

1983 (NAD 83) Many US maps and data sets still use the North American

Datum of 1927 (NAD 27) Differences can be as much as 200 m

Projections and Coordinates

There are many reasons for wanting to project the Earth’s surface onto a plane, rather than deal with the curved surface The paper used to output GIS maps is flat Flat maps are scanned and digitized to create GIS

databases Rasters are flat, it’s impossible to create a raster on a

curved surface The Earth has to be projected to see all of it at once It’s much easier to measure distance on a plane

Geodetic Datums Need a link between:

geoid -- ellipsoid -- sphere

How do we know where locations referenced in the geographic coordinate system are relative to the ellipsoid and geoid?

Geodetic datums provide this link Datum defined: any numerical or geometric quantity

which serves as a reference or base for other quantities a geodetic datum is a reference for mapping

Geodetic Control Network Horizontal datum- has connections

from origin to other points

network of these points at surveyed locations:

geodetic control network

UNC -Chapel Hill - main quad

Datum Info on a Map

Must have this information in order to utilize geospatial data

Why?

Latitude and Longitude

The most comprehensive and powerful method of georeferencing Metric, standard, stable, unique

Uses a well-defined and fixed reference frame Based on the Earth’s rotation and center of mass,

and the Greenwich Meridian

Definition of Latitude

Requires a model of the Earth’s shape The Earth is somewhat elliptical

The N-S diameter is roughly 1/300 less than the E-W diameter

More accurately modeled as an ellipsoid than a sphere

An ellipsoid is formed by rotating an ellipse about its shorter axis (the Earth’s axis in this case)

Geographic Coordinates

spherical coordinate system unprojected! expressed in terms of two angles

(latitude & longitude) longitude: angle formed by a line

going from the intersection of the prime meridian and the equator to the center of the earth, and a second line from the center of the earth to the point in question

latitude: angle formed by a line from the equator toward the center of the earth, and a second line perpendicular to the reference ellipsoid at the point in question

Definition of longitude. The Earth is seen here from above the North Pole, looking along the Axis, with the Equator forming the outer circle. The location

of Greenwich defines the Prime Meridian. The longitude of the point at the center of the red cross is determined by drawing a plane through it and the axis, and measuring the angle between this plane and the Prime Meridian.

Geographic Coordinates

latitude positive in n. hemisphere negative in s. hemisphere

longitude positive east of Prime Meridian negative west of Prime Meridian

Cartesian Coordinates

Computationally, it is much simpler to work with Cartesian coordinates than spherical coordinates x,y coordinates referred to as “eastings” & “northings” defined units, e.g. meters, feet

common examples:

Universal Transverse Mercator: Cartesian coordinate system applicable nearly world-wide

Many countries also have Cartesian systems… U.S. - State Plane U.K. - Ordnance Survey National Grid

Distortions

Any projection must distort the Earth in some way Two types of projections are important in GIS

Conformal property: Shapes of small features are preserved: anywhere on the projection the distortion is the same in all directions

Equal area property: Shapes are distorted, but features have the correct area

Both types of projections will generally distort distances

Cylindrical Projections

Conceptualized as the result of wrapping a cylinder of paper around the Earth

The Mercator projection is the best-known cylindrical projection The cylinder is wrapped around the Equator The projection is conformal

At any point scale is the same in both directions Shape of small features is preserved Features in high latitudes are significantly enlarged

Conic Projections

Conceptualized as the result of wrapping a cone of paper around the Earth Standard Parallels occur where the cone intersects the Earth The Lambert Conformal Conic projection is commonly used to map North America On this projection lines of latitude appear as arcs of circles, and lines of longitude are

straight lines radiating from the North Pole

The Universal Transverse Mercator (UTM) Projection

A type of cylindrical projection Implemented as an internationally standard

coordinate system Initially devised as a military standard

Uses a system of 60 zones Maximum distortion is 0.04%

Transverse Mercator because the cylinder is wrapped around the Poles, not the Equator

Universal Transverse Mercator (UTM)

60 zones, each 6° longitude wide zones run from 80° S to 84° N poles covered by Universal Polar System (UPS)

Universal Transverse Mercator (UTM)

Transverse Mercator projection

applied to each 6° zone to minimize distortion

Universal Transverse Mercator (UTM) Units: meters

Each 6° zone subdivided into North and South zones

N and S zones have separate coordinate systems

x-origin set 500,000m east of central meridian

N zone y-origin: Equator S zone y-origin: 10,000,000m

south of Equator

State Plane Coordinates

Defined in the US by each state Some states use multiple zones Several different types of projections are used by

the system Provides less distortion than UTM

Preferred for applications needing very high accuracy, such as surveying

U.S. State Plane Coordinate System

Each U.S. state composed of one or more zones

Zones trend predominantly N-S or E-W

Each zone has separate coordinate system and appropriate projection

Data Collection

One of most expensive GIS activities Many diverse sources Two broad types of collection

Data capture (direct collection) Data transfer

Two broad capture methods Primary (direct measurement) Secondary (indirect derivation)

Data Collection Techniques

Raster Vector

Primary Digital remote sensing images

GPS measurements

Digital aerial photographs

Survey measurements

Secondary Scanned maps Topographic surveys

DEMs from maps Data sets from atlases

Stages in Data Collection Projects

Planning

Preparation

Digitizing / TransferEditing / Improvement

Evaluation

Primary Data Capture

Capture specifically for GIS use Raster – remote sensing

e.g. SPOT and IKONOS satellites and aerial photography Passive and active sensors

Resolution is key consideration Spatial Spectral Temporal

Typical Reflectance Signatures

Vector Primary Data Capture

Surveying Locations of objects determines by angle and distance

measurements from known locations Uses expensive field equipment and crews Most accurate method for large scale, small areas

GPS Collection of satellites used to fix locations on Earth’s

surface Differential GPS used to improve accuracy

Total Station

Secondary Geographic Data Capture

Data collected for other purposes can be converted for use in GIS

Raster conversion Scanning of maps, aerial photographs,

documents, etc Important scanning parameters are spatial and

spectral (bit depth) resolution

Vector Secondary Data Capture

Collection of vector objects from maps, photographs, plans, etc.

Digitizing Manual (table) Heads-up and vectorization

Photogrammetry – the science and technology of making measurements from photographs, etc.

Digitizer

Tablet Digitizing & Scanning Developing data from analog (paper) maps --

convert information from the analog map into digital form process called digitzing, accomplished using:

tablet digitizer -or- scanner Both approaches require good quality source maps

free of physical distortion (wrinkling, shrinkage) coordinate information visible on map statement of projection, coordinate units, datum, etc.

Tablet digitizing “trace” map from tablet assign attributes

Scanning scan map to create digital “picture” trace picture on-screen or using vectorization software assign attributes

Tablet Digitizer & Software contains fine (.01” - .001”)

mesh of electromagnetically charged wire

common grid resolutions & pucks lead to accuracies ranging from .05mm to .25mm.

Puck- recognizes position on tablet relative to wire mesh.

records coordinates of location tablet in “digitizer units” (e.g. inches, mm).

Digitizing software accepts coordinate information from digitizer& converts from digitizer coordinates to map coordinates.

assembles digitized coordinates into geographic data objects (points, lines, polys).

Digitizing Geographic Features Generally digitize one "layer" (set of related features) from the map at a time

e.g digitize roads separately from hydrography, etc. each digitized set of features becomes a separate vector data layer in GIS

“Trace” the features from the map using the digitizing puck digitize a single x-y location for point feature digitize a series of points to form a line feature

endpoints of lines are nodes points defining shape along lines are vertices digitize a series of lines to form a polygon

Feature digitizing issues: coordinate entry mode:

““point” mode point” mode -vs.--vs.- “stream” mode “stream” mode common polygon borders

treat arcs/lines forming common boundaries as separate entities? or enter common arcs/lines only once? major topological consequences...

Automation During Tablet Digitizing Digitizing is tedious and error prone… Software can “help” by automating

certain steps during digitizing increase efficiency and reduce error Examples: node snapping - automatically join

ends of lines (nodes) together if they fall within a specified distance tolerance

node-line snapping - automatically join end of one line (a node) to an existing line if the node falls within a specified distance of the existing line

intersection detection - automatically detect when two lines cross and create a node at the intersection point

Scan Digitizing

Alternative method for digitizing...

sometimes called “automatic digitizing”

….but it isn’t necessarily very “automatic”

Equipment

scanner "large-format" scanners available as flat-bed or roller scanners scanner "takes picture" of map -- creates a raster image

software

capabilities to read scanned image display image on screen for "heads-up" digitizing or to do

automatic vectorization

Scanner

Factors Affecting Accuracy Source map

inherent spatial resolution of source map (dependent on map scale)

positional & attribute coding errors present in source map

physical condition of map

Digitizing or scanning process care with which map is affixed to digitizing tablet (digitizing)

accuracy of coordinate registration from tablet coordinates (digitizing) or image coordinates (scanning) to real-world coordinates

operator error while digitizing, or while vectorizing scanned image operator error while assigning attribute codes to digitized/scanned

spatial data features

Post-processing effects of generalization, edge matching, rubber sheeting, etc.

Vector to Raster

Raster spatial resolution finer resolution = better representation of the converted

vector data coarser resolution = more information loss!

Method used to determine cell values

How do we know what is “in” each cell?

We choose: cell center (centroid) majority weighting weighted values based on priority/importance

Raster to Vector (Vectorization)

Points & polys - relatively simple points: if cell=value, then a vector point is created

at cell centroid with attribute=value polygons: polygon with attribute=value is created

for all adjoining cells=value; poly boundary follows exterior of cells

Lines - more complex must somehow determine:

start/end/intersection points (nodes) for lines shape points along lines (vertices) topological relationships

Raster to Vector

information loss in result:

Mismatches of Adjacent Spatial Data Sources

Managing Data Capture Projects

Key principles Clear plan, adequate resources, appropriate funding,

and sufficient time Fundamental tradeoff between

Quality, speed and price Two strategies

Incremental ‘Blitzkrieg’ (all at once)

Alternative resource options In house Specialist external agency

TIGER/Line Files

TIGER designed to: support pre-census functions in

preparation for Census of Population and Housing

supports census-taking efforts evaluate success of the Census provide geographic framework for

analysis of Census data

Nominal scale: 1:100,000 Data "layers":

Enumeration units - blocks, block groups, tracts/block numbering areas, counties, cities/MA, etc.; multiple hierarchies

Voting districts; used for Congressional redistricting; Supporting geography

roads/streets/highways basic hydrography point & area landmarks

TIGER Polygon & Landmark Data Point and poly landmarks Census geography (tracts,

blocks, etc.) used for reporting Census data ID linkage from polygons

in TIGER/Line data to Census attribute data

TIGER Line & Address Data

Linear features... Form polygon boundaries Roads

attributes include basic road type, address ranges

also hydro features, etc.

Definitions

Database – an integrated set of data on a particular subject

Geographic (=spatial) database - database containing geographic data of a particular subject for a particular area

Database Management System (DBMS) – software to create, maintain and access databases

Advantages of Databases over Files

Avoids redundancy and duplication Reduces data maintenance costs Applications are separated from the data

Applications persist over time Support multiple concurrent applications

Better data sharing Security and standards can be defined and enforced

Disadvantages of Databases

Expense Complexity Performance – especially complex data

types Integration with other systems can be

difficult

Distributed GIS: Outline

Introduction Distributing the data The mobile user Distributing the software: GIServices

Distributing a GIS

The component parts can be at different locations The user The data The software

The network links all of the parts together

The Role of Standards

Distributed GIS relies on the adoption of common standards To allow the various components to operate

together Such standards have been developed by various

national and international bodies, aided by the Open Geospatial Consortium

Distributing the Data

It must be possible to find remotely located data Data documentation, or metadata, provides the

key to successful search The U.S. Federal Geographic Data Committee

devised a much-emulated standard for geographic data description The Content Standard for Digital Geospatial Metadata

Major Features of FGDC Metadata

1. Identification Information: basic information about the data set

2. Data Quality Information: a general assessment of the quality of the data set

3. Spatial Data Organization Information: the mechanism used to represent spatial information in the data set

4. Spatial Reference Information: the description of the reference frame for, and the means to encode, coordinates in the data set

5. Entity and Attribute Information: details about the information content of the data set, including the entity types, their attributes, and the domains from which attribute values may be assigned

6. Distribution Information: information about the distributor of and options for obtaining the data set

7. Metadata Reference Information: information on the currentness of the metadata information, and the responsible party

8. Citation Information: the recommended reference to be used for the data set

9. Time Period Information: information about the date and time of an event

10. Contact Information: identity of, and means to communicate with, person(s) and organization(s) associated with the data set

Geolibraries and Geoportals

A Geolibrary is a digital library containing georeferenced information Its search mechanism uses geographic location

as the primary key A Geoportal is a digital library of geographic

data and GIServices A one-stop shop for information relevant to GIS

The Mobile User

It is increasingly possible to obtain the services of a GIS through hand-held and wearable devices Some cell phones can be used to generate maps

Such maps can be centered on the user's current location

Map showing WiFi hotspots in the area surrounding the user's current location (the White House, 1600 Pennsylvania Avenue NW, Washington DC)