Electronic Charts: Raster and Vector Formats

DIGITAL SPATIAL DATA

• RASTER

• VECTOR

• Real World

The Basic Difference

• Raster charts are simply bitmap images created by scanning a paper chart

• Vector charts portray charted features as points, lines, or areas with amplifying information (attributes) found in an associated database

Raster Data Definition

8 4 6 12 11

12 2 3 6 0

3 4 9 9 4

8 5 10 3 16

4 7 5 15 10

A data type that stores

information as cell values in a

rectangular grid. A cell is often

referred to as a picture element

(pixel)

6 C C B E

B S B A A

C F E A B

B 5 10 3 16

4 B B 15 10

Integer

Alpha Numeric

0 1 0 1 1

1 1 1 0 1

1 0 0 1 1

1 0 0 1 1

1 0 1 0 0

Binary

0.2 2.1 0.2 1.1 1.6

1.3 1.3 1.4 0.6 1.9

2.1 3.3 0.9 1.4 1.8

8.6 0.4 2.0 2.1 2.9

3.4 2.5 8.1 0.5 2.0

Float

Raster Charts

• A set of colored pixels representing chart information as a picture on computer screen

• Simply an array of pixels arranged in rows and columns

• Pixels are color coded, but do not represent features explicitly

The Appeal of Raster

• Looks like a paper chart

• Cheap and easy to produce (scan existing paper chart)

• Runs easily on PC

• World-wide availability

• An intelligent form of digital geographic data where real world objects are represented by points, lines and polygons. An objects representation is described by attributes and coordinates.

• Examples include:

–Digitized Maps

–GIS Data

Vector Data

Feature Entities

• simple point

• simple line

• simple polygon

• complex

River Spatial Data

Attribute Data

LENGTH WIDTH

2035.00 25.00

ID

1

2 525.00 18.00

#1 #2

• A Feature logically consists of both the spatial data linked

with its attributes in a feature table.

FEATURE ENTITIES

Vector Data

Spatial Data Analysis

•

• •

•

• •

• •

• •

•

2 5

1

4

7 6

Lake

Features:

Line Points:

Connected Detached

Area Text

• Lake

Geographic data are separated

into themes, tiled for speed of

access and placed on CD-ROM Network Analysis

30 30’ N

040 40’ E 30 30’ N

040 50’ E

Vector Topology

Chain# Start Node End Node Left Poly Right Poly

1 1 3 1 0

2 3 4 2 0

3 3 2 1 2

4 1 2 0 1

5 4 2 2 3

6 2 5 0 3

7 4 5 3 0

8 6 6 4 3

2 7 3

1

4

3

5 6

1

2

3

4

8

1 2

4 5

6

0 (“Universe” or “Outside” polygon)

Paradigm Shift

• Looks “different” from a paper chart

• Underlying database allows queries and layer selection

• Zooming reveals detail • Complex, expensive, and

time-consuming to produce • Needs more powerful

computer and expensive display to meet IHO/DoD requirements

Raster and Vector Data Models

Vector Representation

X-AXIS

500

400

300

200

100

600 500 400 300 200 100

Y-AXIS

River

House

600

Trees

Trees

B

B

B B

B

B

B

B G

G

BK

B

B

B

G

G

G G

G

Raster Representation

1 2 3 4 5 6 7 8 9 10

1

2

3

4

5

6

7

8

9

10

Real World

G

G

Raster Model

Land

Shoreline

Water

Vector Shoreline

Raster vs Vector Advantages and Disadvantages

• “Looks like the map.”/ Standard Appearance

• Dumb Map: Very Little Ability for Automated Analysis

• Less Efficient Storage

• Zoom function restricted by pixel size

• Much Easier/Cheaper/ Faster to Produce

• Approximate look of the map/ User-Defined Appearance

• Smart Map: Can Allow Very Powerful Automated Analysis

• More Efficient Storage of Information

• Zoom function not restricted

• Time/Manpower Intensive to Produce

NIMA Raster Products NIMA Vector Products

Summary

• Raster Chart:

–Scanned chart, represented as row and column of pixels

–Cheap and easy to produce and manipulate

• Vector Data:

–Information in the form of points, lines, and polygons w/supporting attribute database

–More difficult to produce and use, but contains more information and capability

–The future of Geospatial Information

ENC, DNC®, or RNC????

Not all charts are equal….

• Many different formats exist for both raster and vector charts

• Most vector charts are digitized paper charts, thereby inheriting any errors (and possibly introducing some!)

• Countries are producing unique digital charts based on their interpretation of IHO standards

ENC

DNC® BSB

ARCS

C-MAP

Navionics

Transas

RNC, ENC, or DNC????

• RNC - Raster Nautical Chart

– “a facsimile of a paper chart originated by, or distributed on the authority of, a government-authorized hydrographic office.” (IHO S-61)

• ENC - Electronic Navigational Chart – means the database, standardized as to content,

structure and format, issued for use with ECDIS on the authority of government-authorized hydrographic offices…” (Sec. 2.2, IMO PS)

– IHO S-57 format

• DNC® - Digital Nautical Chart

– Produced by NIMA in DoD standard Vector Product Format (VPF)

Which is better?

• Depends on your point of view….

• Raster is easier and cheaper to produce, but vector provides much more information and capability to the navigator. Vector is the future.

• NOAA BSB & British ARCs worldwide

• DoD standard is Vector Product Format --DNC®. 5000 charts, but carto source

• ENC produced from original source data

Cartographic Source

Original Source Vector

Data - Image Source