Surveying and Digitizing
Primary Data Sources

Measurements



Remotely sensed data


Field → surveying
Lab (not covered here)
already secondary?
Creating geometries



Definitely in the realm of secondary data
Digitizing
Scanning
Surveying

Measurements and measurement
techniques




Distances
Angles
Position determination
Applications



Traversing and mapping
Construction and earthwork
Boundary surveys
Definition of Surveying

General



To inspect, view, scrutinize, or examine
To determine condition, situation, or value
Specifically

Science and art of determining relative
positions of points above, on, or beneath
earth surface
Uses of Surveying

Locate/map resources

Engineering design




Layout construction or engineering projects
Verify performance
Acquire reliable data
Provide control

Usually for location
History of Surveying

Early applications




Boundary location
Construction
Mapping
Early surveys limited by technology


Crude and inconsistent methods
Development of sighting devices,
standards, …
History of Surveying (2)

Industrial revolution improved surveying



Advances in available materials
Improvement in tools
Electronics revolution  fundamental
advances



Electronic distance and angle measurement
Satellite surveying
Enhanced processing
Specific Types of Surveying







Property (cadastral) surveying
Control surveying
Mapping surveying (planimetric or
topographic)
Photogrammetric surveying
Construction (engineering) surveying
Route surveying
Hydrographic surveying
Surveying Measurements

Two quantities measured in surveying
Lengths
 Angles


All measurements are imperfect
Errors
 Mistakes

Measurement Errors

Sources of errors




Personal
Types of errors



Natural
Instrumental
Systematic
Random
Terms used in describing errors


Precision
Accuracy
Idea of Relative Position



Question: Have the points moved?
Answer: Relative to what?
References


Needed for expressing location of points,
lines, other objects
Datums provide references in surveying


Horizontally
Vertically
Reference Ellipsoids
Basic Concept
a = semi-major axis
b = semi-minor axis
f = flattening
b ab
f  1 
a
a
e = eccentricity
a 2  b2
e
 2f  f 2
a
Example Reference Ellipsoids
Ellipsoid Equatorial Axis
Polar Axis
Association
Clarke, 1866 12,756,412.8 m 12,713,167.6 m NAD27 datum
GRS80
WGS84
ITRS
12,756,274 m
12,713,504.6 m NAD83 datum
12,756,274 m
12,713,504.6 m GPS
12,756,272.98 m 12,713,503.5 m ITRF
GRS = Geodetic Reference System
WGS = World Geodetic System
ITRS = International Terrestrial Reference System
Ignoring Earth Curvature

Distance
8000.000m ( 5 miles)
1000 km
 998.95 km
Ignoring Earth Curvature (2)

Level line
Horizontal plane
1 mile (1609 m)
8 inches ( 20 cm)
Level surface
Ignoring Earth Curvature (3)

Triangle geometry
mi2
75
(48,000 acres)
19,800 hectares
Sum of Interior
Angles =
180° 00' 01"
Digitizing and Scanning

Instruments

Georeferencing

The process and problems associated
with it

Automation

Formats
Why Do We Have To Digitize?



Existing data sets are general purpose,
so if you want something specific you
have to create it
In spite of 20+ years of GIS, most stuff
is still in analog form
Chances are somebody else has
digitized it before; but data sharing is
not what it should be
Digitizer


Digitizing table
10” x 10” to 80” x 60”
$50 - $2,000
1/100th inch accuracy
Stylus or
puck with control buttons
The Digitizing Procedure

Affixing the map to the digitizer

Registering the map

Actual digitizing


In point mode
In stream mode
Georeferencing



Entered:
at least 3 control points
Tic 1: 11° 15' N
30° 30' E
aka reference points or tics
Tic 2: 11° 15' N
73° 30' E
easily identifiable on the map
exact coordinates need to be known
East of Greenwich
Digitizing Table Coordinates
71°
72°
73°
11°
11°
12°
12°
South
Origin:
X = 4 in.
Y = 5 in.
Tic Points
71°
72°
73°
Digitizing Modes

Point mode





most common
selective choice of points digitized
requires judgment
for man-made features
Stream mode



large number of (redundant) points
requires concentration
For natural (irregular) features
Problems With Digitizing

Paper instability

Humidity-induced shrinking of 2%-3%

Cartographic distortion, aka
displacement
Overshoots, gaps, and spikes

Curve sampling

Errors From Digitizing


Fatigue
Map complexity

½ hour to 3 days for a single map sheet

Sliver polygons

Wrongly placed labels
5
6 7
8
Digitizing

Rule of thumb: one
boundary per minute
ergo:
appr. 62 lines
= more than one hour
Costs
Automated Data Input
(Scanning)


Work like a photocopier or fax machine
Three types:

Flatbed scanners




Drum scanner



A4 or A3
600 to 2400 dpi optical resolution
$50 to $2,000
practically unlimited paper size
$10k TO $50k
Video line scanner

produces
vector data
Requirements for Scanning




Data capture is fast but preparation is tedious
Computers cannot distinguish smudges
Lines should be at least 0.1 of a mm wide
Text and preferably color separation
300


AI techniques don’t work (yet?)
Symbols such as  are too variable for
automatic detection and interpretation
Semi-automatic Data Input
(Heads-up Digitizing)

Reasonable compromise between
traditional digitizing and scanning

Much less tedious

Incorporating your intelligence
Criteria for Choosing
Input Mode


Images without easily detectable line
work should be left in raster format
Really dense line work should be left as
background image –

unless it is really needed for automatic GIS
analysis; in which case you would have to
bite the bullet
Conversion from Other
Databases



Autocad .dxf and dBASE .dbf are de facto
standards for GIS data exchange
In the raster domain there is no
equivalent; .tif comes closest to a
“standard”
In any case: merging data that originate
from different scales is problematic – in
the best of all worlds; there is no
automatic generalization routine