gis applications in civil engineering note #3 maps as numbers gis data structures xudong jia, ph.d.,...

GIS Applications in Civil Engineering

Note #3 Maps as Numbers GIS Data Structures

Xudong Jia, Ph.D., P.E.

January, 2011

Summary of the First Two Lectures

How do we describe geographical features?by recognizing two types of data:

Spatial data which describes location (where)Attribute data which specifies characteristics at that location (what, how much, and when)

How do we represent these digitally in a GIS?by grouping into layers based on similar characteristics (e.g hydrography, elevation, water lines, sewer lines, grocery sales) and using either:

vector data model (coverage in ARC/INFO, shapefile in ArcView)raster data model (GRID or Image in ARC/INFO & ArcView)

by selecting appropriate data properties for each layer with respect to: projection, scale, accuracy, and resolution

How do we incorporate into a computer application system?by using a relational Data Base Management System (DBMS)

Maps as Numbers

Maps are represented by using numbers

Binary and Decimal System. How to convert a binary value to a decimal number or vice versa.

Hexadecimal number 00 to FF in one byte

More examples on binary and decimal conversions.

Maps as Numbers

Two data models for maps : Vector and Raster

One data model for attributes: flat files

Cell size determines the resolution of the data.

The grid has an extent.

Each grid is owned by one feature

Features cannot be perfectly fitted into cells.

Cell’s bit depth provides a range to describe possible feature values.

Raster: It is a model that contains a grid. Each grid cell is a map unit used to represent a pixel.

Maps as Numbers

Benefits of using Raster Model

Easy to understand

Capable of rapid retrieval and analysis

Easy to draw on the screen and on computer devices which display pixels.

Maps as Numbers

Vector Model

Composed of points that are provided by exact coordinates

Accuracy

Efficient at storing features

Able to store topological attributes for features.

Maps as Numbers

Structuring Attributes

A flat file as show on Page 84 of the Textbook

Relational database table

A distributed database through server extensions, database engines, and data warehouse.

Data dictionary or metadata, the list of all the attributes along with all their characteristics save as a separte file or as a header of the file

Maps as Numbers

Structuring Maps

Vector Data Structures

Cartographic Spaghetti

Maps as Numbers

Structuring Maps


Arc/Node Model

Maps as Numbers

Structuring Maps


Topological Arc/Node Model

(see Page 88)

Maps as Numbers

Structuring Maps


Triangulated Irregular Network (TIN Model)

Maps as Numbers

TIN: Triangulated Irregular Network Surface

A B

CD

6

12

3

4

5

E

F

GH

Elevation points (nodes) chosen based on relief complexity, and then their 3-D location (x,y,z) determined.

Node # X Y Z1 0 999 14562 525 1437 14373 631 886 1423

etc

PointsPolygon Node #s Topology

A 1,2,4 B,DB 2,3,4 A,E,CC 3,4,5 B,F,GD 1,4,6 A,H

etc

Elevation points connected to form a set of triangular polygons; these then represented in a vector structure.

PolygonsPolygons Var 1 Var 2

A 1473 15B 1490 100C 1533 150D 1486 270

etc.

Attribute Info. Database

Attribute data associated via relational DBMS (e.g. slope, aspect, soils, etc.)

Advantages over raster:•fewer points•captures discontinuities (e.g ridges)•slope and aspect easily recordedDisadvans.: Relating to other polygons for map overlay is compute intensive (many polygons)

Maps as Numbers

Raster Data Structure

Challenging issues on mixed pixel problem, redundant or missing data and large storage requirement

Maps as Numbers

Run Length EncodingFull Matrix--162 bytes

111111122222222223111111122222222233111111122222222333111111222222223333111113333333333333111113333333333333111113333333333333111333333333333333111333333333333333

1,7,2,17,3,181,7,2,16,3,181,7,2,15,3,181,6,2,14,3,18 1,5,3,18 1,5,3,18 1,5,3,18 1,3,3,18 1,3,3,18

Run Length (row)--44 bytes

Now, GIS packages generally rely on commercial compression routines. Pkzip is the most common, general purpose routine. MrSid (from Lizard Technology)and ECW (from ER Mapper) are used for images. All these essentially use the same concept. Occasionally, data is still delivered to you in run-length compression, especially in remote sensing applications.

This is a “lossless” compression, as opposed to “lossy,” since the original data can be exactly reproduced.

Maps as Numbers

R-Tree Encoding

See Page 92

Maps as Numbers

Quad Tree Encoding See Page 92

• sides of square grid divided evenly on a recursive basis

– length decreases by half– # of areas increases fourfold– area decreases by one fourth

• Resample by combining (e.g. average) the four cell values

– although storage increases if save all samples, can save processing costs if some operations don’t need high resolution

• for nominal or binary data can save storage by using maximum block representation

– all blocks with same value at any one level in tree can be stored as single value

Layer Width Cell

Count1 1 12 2 43 4 164 8 645 16 2566 32 1024

store this quadrantas single 1

store this quadrant as single zero

1 1

1 1

1 1

1

1

Essentially involves compression applied to both row and column.

2

2

1

2

3

4

4

4

4

54

4

4

3

4

2

3 4

2.53.5

3.25

Maps as Numbers

Image Pyramid and Quad Encoding

See Page 92

Maps as Numbers

Why Topological Matters

Maps as Numbers

Why Topological Matters Terms: SliversSpikeUnsnapped NodesUn-ended lines

Maps as Numbers

Formats for GIS Data

Vector Data Formats:

HPGL – Page Description LanguagePostScript - Page Description LanguagePDF – Portable Documen Format , GeoPDFAutoCAD DXF formatDIME/TIGER FormatDLG FormatKML GMLXML

Maps as Numbers


DIME/TIGER Data Model

Maps as Numbers


DIME/TIGER Data Model (Page 100)

Maps as Numbers


DLG Model

Maps as Numbers

Raster Data Formats

TIFGIFJPEFGEOTIPPNGEncapsulated PostScriptDEM (1:24,000 and 1:100,000)

Maps as Numbers

Exchange Data and Data Standards

Data exchanges between Raster and Vector Data exchanges among different GIS platformsData exchanges between different systsems

SDTS (Spatial Data Transfer Standards)Open Geospatial Consortium Open GIS Specifications

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