Download - Spatial Data Formats

CS 128/ES 228 - Lecture 4b 1

Spatial Data Formats


Need data for a GIS?

• Just photograph a topographic map

• Better yet, download one from the internet

• But are the roads, buildings, and other “objects” on this photo GIS layers?


Stages of development:1. Conceptual model: select the features of

reality to be modeled and decide what entities will represent them

2. Spatial data model: select a format that will represent the model entities

3. Spatial data structure: decide how to code the entities in the model’s data files


2. Spatial data models

1. Raster

2. Vector

3. Object-oriented

and… attribute data

Spatial data formats:

Fig. 3.1 in 3rd ed.


Raster format Features

represented by cell contents

Spatial precision limited by cell size

Surfaces modeled as continuous values (almost)

Fig. 3.9 in 3rd ed.


Vector format Discrete features

explicitly represented

Spatial precision limited by number format

Surfaces shown by contours rather than continuous values

Fig. 3.9 in 3rd ed.


Object-oriented formats

Leave details for CS majors

Fig. 4.17 in 3rd ed.


Thematic data (a.k.a. “attribute data”)

Quantitative or descriptive

May represent 1 or many themes

Tied to a spatial reference

Represented differently in raster vs. vector formats


Scales of measurement

Data Unit ScaleResort name text Nominal

Resort ranking value Ordinal

Winter temp. oC Interval

Size of ski area m2 Ratio

Heywood et. al. 2006 – Table 2.1


Spatial modeling in raster format Basic entity is the

cell

Region represented by a tiling of cells

Cell size = resolution

Attribute data linked to individual cells


Attribute data in raster formatAttribute data are used to create symbology for

each cell


Additional attribute data Some GISs provide a VAT linked to individual

cells (e.g. ArcInfo GRID)

VAT data then accessible to database management system

Unlimited additional fields


Attribute data for a vector layer Each entity is linked to a row in an attribute

table

Themes not (usually) displayed but available via Identify tool

CS 128/ES 228 - Lecture 4a 14

Vectors are good at modeling …

roadways or wiring consist of discrete components; types and order of the connections are key

Spaces between the network components generally not of interest Bottom : http://www.dunereview.com/electricalupgrade-1.htm

… networks


Rasters are good at modeling …

they model a continuous feature as a 2- or 3-D layer

every location has a value, even if only interpolated from discrete samples

Both: http://snobear.colorado.edu/Markw/Research/ESRI/ESRI.html

… surfaces


Topographic maps use contours…

…but the elevation between contour lines is undefined


Digital terrain models

Every cell has an elevation value



Precision agriculture

Aerial photograph Soil pH Crop yield


Oceanography

Estimate of phytoplankton distribution in the surface ocean: global composite image of surface chlorophyll a concentration (mg m-3) estimated from SeaWiFS data (Source: NASA Goddard Space Flight

Center, Maryland, USA and ORBIMAGE, Virginia, USA).


Rasters are a type of Tesselation A closed shape or polygon that repeats on all

sides without any gaps or overlaps

Three regular polygons tesselate the plane:

Square Equilateral triangle Hexagon


TilingsIn 1922 Escher visited the Alhambra palace and saw the wall tilings of the Moors. He was excited to find other artists who had been captivated by tilings, but also made this revealing comment: "What a pity their religion forbade them to make graven images."


Escher’s “tesselations”


Quilters also tesselate

Designing Tesselations by Jinny Beyer


Effects of resolution – rasterLarger cells: less precise

spatial fix

line + boundary thickening

features too close overlap - less detail possible



Advantages of raster format many data sets

available

easy to overlay multiple themes

able to represent multiple continuous surfaces

different file formats readily inter-converted

fast computer lookup and display


Limitations of raster format poor representation of

discrete objects

exact boundary location difficult

constant resolution throughout the region modeled

generates very large data sets

difficult to change projection or coordinate system


Raster layers don’t share well


Raster layers are normally projected•Note the datum and projection/ coordinate system

• Special software needed to re-project


Summary: Raster format Huge amounts of

spatial data are available in raster format

Rasters are the format of choice for continuous features

Rasters do a poor job of representing discrete features