GIS FINAL EXAM

Projecting GIS data & map projections

Plannimetric coordinates - systems that are expressed

in measures of x and y values.

Geographic coordinates - systems, on the other hand,

are expressed in values of longitude and latitude.

Projection – A process that involves conversion of data from Geographic

Coordinates, Lat Long to projected coordinates.

Reprojection – Converts from one projected coordinate system to another (e.g.,

Michigan State plane to UTM).

Meridians – Lines of equal longitude. Begin at 0 degrees,

starting in Greenwich, England. Goes 180 degrees West and East;

Parallels – Lines of equal latitude. The equator forms the 0 degrees

and the poles are a 90 degrees. Measurements in N or S direction.

Degrees-Minutes- Seconds (DMS) = Angular measures of location;

One degree = 60 Minutes; One Minute = 60 Seconds;

46° 33' 42" N, 87° 24' 26" W = “46.5616666, -87.40720000"

EARTH’s SHAPE = A geoid (spheroid, ellipsoid) provides a

‘precise’ model of this shape. The Earth is slightly flattened

(due to centrifugal force and the Earth’s rotation)

Datum - mathematical model of the Earth, serving as a reference for calculating

geographic coordinates. initially derived using ‘ground-based’ manual

measurements (e.g., Clarke 1866) current models use satellite based

measurements.

If different datum are used to project the data a point can change in position from

10-100meters in communicated location! (e.g., NAD83 vs NAD27 vs. WGS84)

MAP PROJECTIONS - the process of transforming a spherical (well technically a

geodetic surface) surface onto a flat surface or plane (i.e., plannimetric surface).

3 Main Types of Projections;

• Conformal – Preserves angles and shapes;

• Equidistant – Preserves areas;

• Azimuthal – Retains certain accurate directions (flat plane is fix to a

single point on the earth usually a polar projection);

Conformal Equidistant Azimuthal

UTM - (Universal Trans-Mercator) Used for global purposes.

US State plane system – Used for individual states purposes.

(Michigan State Plane Projected coordinate system)

There are thousands of coordinate systems to

use on GIS to suit your mapping area / purpose.

Commonly Used Projections

Transverse Mercator - best known for mapping the world and is a

conformal projection.

Lambert Conformal Conic – A conical projection used mainly for

mapping East-West stretches at mid-latitudes due to it’s conical shape and

position minimizing distortion in these areas.

Albers Equal Area Conical – Similar to the Lambert except the Albers

Equal Area projection, as the name suggests, offers an equal area projection

as opposed to the Lambert system, which is conformal and preserves angles

and shapes.

Transverse Mercator Lambert Conformal Conic Albers Equal Area Conical

Projection on the fly - a term that is used by the software companies. It implies

data with different projection systems can be displayed simultaneously. Most of

the time things work fine… until your data arrive without a projection system. A

default system is prescribed. This default system may not match what the data

were mapped in originally and problems occur…

Creating GIS Data & Digitizing

Remotely sensed data - Satellite imagery: useful for land

cover datasets; vegetation health, crop inventories, soil,

geologic features and ocean or lake temperature data.

- Higher resolution sensors: Quickbird and Ikonos helping provide precise

datasets and map areas in detail.

Field data

• GPS data, a familiar source for distances, elevations;

• Collected with hand/based surveys;

• GIS datasets are constantly updated using information

collected using GPS equipment.

Digitizing – process of converting data from antilog to digital format.

Digitizing tables – rarely used anymore, but they are a

board with an electronic mesh that can sense the position

of a cursor. A paper map is registered to the tablet and the

mouse (cursor) traces the contents of the paper map to

the digital environment;

Scanning - is also a form of digitizing and converts an analog map to a scanned

file.

‘vectorization’ converting the raster lines into vector lines or areas for used in

GIS vector datasets (shapefile, geodatabase etc.).

‘heads up digitizing’ involves using a scanned image or a scanned document

displayed on a computer monitor as a ‘back-drop’ data source; Vector lines are

traced over the image as a way of entering in the data.

The GeoDatabase:Object based vector data model Coverages and Shapefiles are the forerunners to the latest development, the

Geodatabase data model.

• Coverages 1980s (arrived with ArcInfo);

• Shapefiles 1990s (arrived with ArcView);

• Geodatabase 1999(ish) arrived with ArcGIS version 8

Geodatabases are the most sophisticated spatial data format of these three

formats. GeoDatabases take advantage of object based technologies with a wider

range of options as they are maintained and built by Database systems,

Automatically maintains and updates topological relationships.

• Points = 1 or more points;

• Polyline = set of line segments… not always connected;

• Polygon = 1 or more rings or areas. Rings are closed connected lines.

Feature class: Stores spatial data of the same geometric type;

Feature dataset: stores feature classes that share the same coordinate system

and extent. May contain different theme layers within the same file.

Spatial features are represented as separate objects

(e.g., a timber stand = one object) all the geometric shapes

and attributes of the stand are stored in a single record; thus

eliminating the need for the split file system.

Enterprise GIS - This multi-user setting can become

centralised database for an organisation and many

users can access the information simultaneously,

with various levels of accessibility. This multi-user

centralized Database system is the fundamental

concept of an Enterprise GIS.

• Versioning allows multiple users to edit the same data in an ArcSDE

geodatabase (only) without applying locks or duplicating data.

• Archiving allows users to manage past changes sort of a temporal GIS.