Tech Con Env - ch 2 - spatial data
Intro - by definition, GIS are just models of real world
Usually contain just the data you need to solve your problem
This simplified view known as a model of real world
A. Look at character of the spatial data used in the model
- Differentiate between data and information
Data are the raw "facts" that are processed to become information
e.g., data as raw numbers in a notebook, men a nothing until you realize
guy is measuring snowpack in inches at diff locations
so you need to know:
1)what is being measured and
2) how (the units of measurement)
in order for the data to become information
2 types of data - primary (first-hand observation)
secondary (second-hand, from a source other than you)
but all data have 3 "modes" or "dimensions"
- Temporal (timing)…WHEN
- Thematic (type of data, the theme; the attribute)…WHAT
- Spatial (where it's collected or measured)…WHERE
One dimension is usually used to organize the data, but the other data are
displayed too (like a sort function in Excel…you sort by one attribute, but all
the other data "go along for the ride"
"great emphasis" in GIS is using the "spatial dimension" for turning data
into information
So now let's turn to characteristics of spatial data
B. Maps & influence on character of spatial data
- Storage - analysis - presentation of data is usually the map's job
- Number of steps accomplished during the mapping process
 Define purpose
 Define scale
 Select features to portray
 Choose representation (pt, line, polygon)
 Adopt map projection (when not staying in lat-long)
 Annotate w/ legend, etc.
Ch 2 p.2
Let's examine the steps in detail…
 Purpose - several examples
- Map of ski area to help people find way around, spend money!
- WW II map of Allied occupation of Europe, distorted to make
Allies look good.
- Topographic maps to show many themes.
Purpose also intertwined with scale….
 Scale - simplest definition - ratio distance on map
distance on ground
example: 1: 24,000 is common scale
1" on map represents 24,000" on ground (24,000" = how many feet??)
ratio, verbal, and graphic scales are all common
ratio is 1:24,000
verbal is 1" = 2,000 feet
graphical is
0
2,000 feet
the graphical and ratio scales are very popular in GIS, because scale is
automatically updated as you change your view….there would be a big
problem if you tried to use a verbal scale, because 1" would represent
different distances on ground every time you change your view.
 Spatial entities - symbols (pts lines poygons) that represent real-world
features are the "basic spatial entities".
Recognize that the use of these symbols is VERY scale dependent - Cities could be dots or areas
- Roads could be lines or areas
 Generalization - some is inevitable, because we are dealing with
smaller-than-real situations fig 2.2 shows the changes in
detail/generalization that accompany changes in scale
Ch 2 p.3
 Projections Why do these? Because we need them to go from 3D spherical world to
2D map world.
All projections introduce some distortion, but you pick your poison…some
keep distance accurate, but direction suffers. Or shape is preserved but
area is not.
Analogy - inflated beach ball is like a globe…think about deflating it and
spreading it flat on a table (like any 2D surface)….very difficult to
represent 3D accurately in 2D….
Fig 2.3 - Best way to visualize….place a light bulb inside a globe, look at
the patterns on wall or ceiling of a room, or cylinder wrapped around globe.
Note that any projection has least amt of distortion at center of image.
Cylindrical projection, wrapping around equator…least distortion at the line
of contact at the equator, the "line of true scale". This is where distance
(and thus scale) is least distorted.
Out at the poles, things very distorted…
With azimuthal projection on flat wall, not cylinder, distortion occurs out at
all 4 edges, not just 2.
Cylindrical
Azimuthal
Low distort
Low distort
Conic , wrapped like the cylinder but in a tepee, so the low distortion is
along line of contact like with the cylindrical projection.
Low distort
Ch2 p.4
Cylindrical projections include Mercator (famous). Transverse Mercator
(wraps around the prime meridian) preserves
 Scale
 Shape
 Area
 Direction (bearing)
for small areas….Which is why it's so popular; does a lot of things pretty
well.
Projections in GIS go hand-in-hand with spatial referencing issues
associated with them.
 Spatial referencing
Referencing system required to locate features in space. System should
have certain capabilities:
- Stable (that is, reliable)
- Can show pts, lines, polygons
- Can measure length, area, shape
3 main means of referencing:
1. geographic coords
2. rectangular coords
3. non coord systems
1. Latitude/longitude are the only TRUE geographic coords..this is the only
means to truly locate a feature on a sphere
Brief description/reminder - we measure in degrees/minutes/seconds (60
secs in a minute, 60 minutes in a degree).
Lat starts zero at equator, goes to 90o N at north pole, 90o S at south pole
Long starts at Prime Meridian through Greenwich, England, goes to 180o E
and 180o W.
Again, lat/long is the only TRUE geographic coord system, because it's the
only one that works on a sphere. Note: USGS topo maps are defined
according to degrees/minutes/seconds (7.5' or 15' quads; 1o x 2o sheets)
Much of the default data in Arcview is in lat -long….AV claims that in small
areas, lat-long is fine for 2D maps. But another system, using XY coords
like a Cartesian graph, is known as rectangular coords.
Ch 2 p.5
2. Much data in GIS is in rectang coords. So how does this happen?
Using a projection system, use the lines of lat and long to set up a grid
With a small area, the grid lines are not offset. But with larger areas,
like over a state, grids must shift.
UTM (universal transverse mercator) grid used many places, including
US and Grt Britain. Earth divided into 60 zones of longitude, each 6o
wide.
3. non-coord systems - use a descriptive code, like a zipcode. Another
example is Public Land Survey System, which is
section/township/range system.
 Topology
How features are connected to or relate to each other - 3 elements:
1. Adjacency - common boundary between features
2. Containment - one feature within another
3. Connectivity - linkages between line features
Topology important for answering questions like, "what is shortest or
fastest distance form pt A to pt B.
C. thematic characteristics of spatial data
"thematic" refers to the themes or "attributes" of these data..they are the
characteristics of the entities that are represented by points, lines,
polygons
of course there can be MANY attributes associated with an entity…the list
is limited only by your imagination….remember the example of a tree? A
lake? A road?
These attributes can be easily stored in a table or database that
looks just like an Excel spdsht or Access database. But for the attributes to
have meaning, you have to know what their "units" are, which relates to the
"scale of measurement".
4 basic scales are: NOIR
 Nominal - just for naming or describing ("green")
 Ordinal - for ordering ("2nd largest")
 Interval - the difference between numbers is important, but no common
zero (example: oC scale…20oC NOT twice as warm as 10oC)
 Ratio - all numerical operations are valid; common zero
Ch 2 p.6
Take a couple examples from the book. Use the Happy Valley Ski resort.
These may all be numbers, but they mean different things….
Nominal - numbers for IDENTITY - like phone number, ski pass, even your
Soc Sec number - "cannot be processed in a mathematical sense"
Ordinal - numbers for ORDER - your number in line for a ski lift - the top 10
cafes by attendance
Interval - numbers show MEANINGFUL DIFFERENCES, but without a real
zero or starting point, can't do full range of math operations. Example,
ratio for 20oC / 10oC cannot be 2….the numerator is NOT twice as much as
th3e denominator. Negative values are possible.
Ratio - numbers for RATIOs are possible. So the full range of oprns is
possible. Negative values are NOT possible…zero is as low as you can
go. Think of 0oK - absolute zero, all molecular motion ceases to exist. So
the Kelvin temp scale is ratio, but Fahrenheit and Celsius are not.
BIG problem to watch for in GIS - many numbers in GIS are really nominal
or ordinal, but there's tendency for folks to perform numerical operations
with these values, and assign some sort of numerical significance to the
outcome. This is totally wrong and misleading…if there is ONE thing you
get out of this course, it is to ALWAYS be on the lookout for this type of
manipulation, and do NOT trust the results or outcomes without evaluating
the procedure and logic YOURSELF
C. Other sources of spatial data
Maps alone do not represent all the sources of spatial data we can
access..others include:
 Census/survey data….can be considered spatial if we can locate the
source of the data in space (e.g., the house, apt bldg, street, city, etc.).
Examples: population census, agricultural census, marketing data
 Air photographs - snapshot of Earth at a given instant in time. Really
good for "monitoring change". Low cost relative to other remote
sensing data. Problems involved with scale, but can be overcome by
rectifying the photo to maps or by using GPS data.
 Remote sensing / satellite images - really at the heart of GIS revolution
- so much data available from satellites, we needed computer capability
to handle the data.
Ch 2 p.7
Sensors on board the satellite collect data, relay it to Earth. Sensors
detect radiation emitted from Earth, much of which is not in visible light
portion of spectrum.
Resolution of satellite images often in pixels 100' x 100' on a side, so they
are useful for large-scale work, large areas being covered. But this pixel
size too big for highly precise work.
Lots of advantages to using remote sensing images in GIS.
 Digital format very compatible with GIS
 Conducive to processing in GIS
 Excellent for regional studies of geology, environment, etc.
 Direct data collection in the field: surveying and GPS
Raw data another important data source. Traditional survey methods were
pencil and paper, but more modern instruments utilize data loggers that
automatically store info in spreadsheet-style format (like x,y,z coordinates
from a total station).
Latest positioning from GPS (global postioning system) FIG 2.9 shows the
concept….essentially handheld satellite receivers that can very accurately
locate your position if you can get a fix from several satellites at once.
GPS rapidly gaining popularity..easy to use, potential to be very accurate.
Who's seen one of these in a car? More and more….
Summary of chapter:
 GIS models only as good as the data used to construct them.
 Need to know the characteristics of these data.
Next step - how do we store these data so we can acces them with GIS?