Census Mapping with GPS
Natalie Dobbs
Spatial Innovision Limited
Tech@spatialvision.com
1
Overview
• Case Study
- Municipality of Rome
• Introduction to GPS
• Mobile GIS
– Terrasync Workflow
– ArcPad Workflow
• Trimble Field Computers with GPS
Receiver
• Accuracy
• Conclusion
2
Case Study
MUNICIPALITY OF ROME:
The Integrated Territorial System
OFFICE OF STATISTICS AND CENSUS
SERVICE OF TERRITORIAL DATA
BANKS
3
Case Study: Field Activity
• The main activities carried out in the field
using portable devices include:
a) Collection of field data:
- Update of attribute information
- Maps used as background images allowed
visualisation of activity
- GPS support enabled georeferencing of
collected field information instantaneously.
4
Street Graphic Layers - Satellite
Image
5
Case Study: Field Activity
cont’d
b) Field Inspections :
- GPS enabled the collection of assets and
resources (schools etc.)
6
Case Study: Summary
• Reliability of the information contained in
the GIS has a key importance.
• Data quality influences:
- Specific planning and management of
the area
- Resulting in either a more or less
positive result which is important to a
local authority
7
Sphere of Influence of a School
to Determine Demographic
Capacity
8
What is GPS?
• The Global Positioning System (GPS) is
- Constellation of at least 24 satellites
- Develop by the United States Department of
Defense (DoD)
- Used worldwide by any civilian free of charge
9
GPS Segments
• Divided into three segments
1) Control
- Receives transmission &
transmit information
- Operated by the U.S. Department of
Defense
2) Space
- 24 satellites:
- Orbit 20,200 km above the Earth
- 1 revolution in 12 hours
3) User
- Utilized by many application of GPS to
calculate GPS positions
- Civilian use outnumber military use
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Why We Use Satellites for Mapping
• Line of Sight
11
Satellite Ranging
• Measuring the distance from a satellite
•Measuring travel time of radio signals
Distance D = Time x Speed of light (300,000
km/sec)
12
Trilateration From Satellites
• By measuring distance from several
satellites you can calculate your
position
Trilateration
• One measurement narrows down our
position to the surface of a sphere
11,000 miles
We're somewhere
on the surface of
this sphere.
Trilateration
• Second measurement narrows it
down to intersection of two spheres
Intersection of two
Spheres is a circle
Trilateration
• Third measurement narrows to just
two points
Trilateration
• Fourth measurement will decide
between two points
Source of Errors
• Atmosphere Error
- GPS signals are delayed as they pass through the
atmosphere
- The signals are delayed by the ionosphere and
troposphere
Ionosphere
Troposphere
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Source of errors cont’d
• Multipath Error
- Occurs when GPS
signal is reflected off
an object before it
reaches the GPS
antenna
• Selective
Availability
- U.S. Department of
Defense introduced
artificial errors to
reduce GPS position
accuracy
- Was largest source
of error
- Turned off May
2000
19
Source of Error cont’d
• Obstruction
20
Differential GPS
Two receivers track SAME signals
and errors at SAME time
21
Comparing data acquisition
technologies
• GPS is one of many technologies
– Manual digitizing of paper maps
– Collection of attribute data with pen and
paper
– Scanned paper maps
– Aerial photography
– Satellite imagery
– LIDAR or other 3-D scan imagery
22
Comparing data acquisition
technologies
• GPS especially suitable for
-
Features where attributes only
determined by field inspection
Features where other geocoding
reference (e.g. street address) is
absent
Integration with GIS as data is digital
Data where positional accuracy is
critical to subsequent decision
making
Navigation to and identification of
assets by position
Scenarios where data use in the field
is planned
23
Mobile GIS
24
Mobile GIS
• Expansion of GIS technology from the office
into the field
• Integration of three essential components:
- Global Positioning System (GPS)
- Field Computer
- GIS field software :
- Data collection and maintenance
- ArcPad and Terrasync
• Two suggested workflow:
- Terrasync workflow (Trimble)
- ArcPad workflow (ESRI)
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Field Software
• Open platform to run
the GIS software that
you require
• Support the data
schema of GIS
– Data dictionary or
Native Shape files
– Observe the domain
rules of the GIS
• Display raster and
vector in the field
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Terrasync Workflow
Create Data Dictionary
using GPS Pathfinder
Office Software
Transfer Data
Dictionary
using Data
Transfer Utility
Postprocess
data in GPS
Pathfinder
Office
Software
Transfer Data
Dictionary
using Data
Transfer Utility
Terrasync
Software on
Field Computer
with GPS
Receiver
Export to
GIS or CAD
format
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ArcPad Workflow
• Creation of a personal geodatabase
• GPS- enable the personal geodatabase
• Check out data from the Personal
Geodatabse and transfer it to the field
device
• Collect data using ArcPad and GPScorrect
extenion
• Check in data to field computer
• Process the imported GPS data with the use
of GPS Analyst
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ArcPad Workflow
check out
GPS Data
ArcGIS
ArcPad
GPS Analyst
GPScorrect
check in
• Differentially correct
• Query and analyze GPS data
• Validate GPS position accuracy
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Trimble Field Computers with GPS Receiver
GeoXH
PostProcessed
Accuracy
Subfoot
<30 cm
GeoXT
GeoXM
Juno
ST
XB
Edition
XC
Edition
Submeter
<1 m
1-3 m
2-5 m
2-5 m
2-5 m
Everest Multipath
Rejection
Technology
Yes
Yes
No
No
No
No
H-Star Technology
Yes
No
No
No
No
No
Bluetooth
Yes
Yes
Yes
Yes
Yes
Yes
Integrated
wireless LAN &
TrimPix
technology
Yes
Yes
Yes
Yes
Yes
Yes
Windows Mobile 5
Yes
Yes
Yes
Yes
Yes
Yes
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Trimble Field Computers with GPS Receiver
Cont’d
GeoXH
GeoXT
GeoXM
Juno ST
XB
Edition
XC
Edition
Logging
Memory
512 MB
512 MB
512 MB
128 MB
128 MB
128 MB
Expandable
Memory
1 SD slot
1 SD slot
1 SD slot
1 SD
slot
2
Compact
Flash
slots
1
Compact
Flash
slots
Battery Life
9 hours
9 hours
9 hours
6 hours
8 hours
8 hours
Ruggedness
Withstand
s 1m (3.2
ft) drop
Withstand Withstan
s 1m (3.2 ds 1m
ft) drop
(3.2 ft)
drop
Nonrugged
Recon
handhel
d:
withstan
ds 1.2 m
(4 ft)
drop
Withstan
d 1.2m
(4.4 ft)
drop
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GPS requirements – Accuracy
in the Office
• What analysis will be made with
the data?
To uniquely identify dwelling
requires 3 m GPS accuracy
Digging up a pipe requires
<30cm GPS accuracy
• What is the accuracy of other GIS
layers?
32
GPS requirements – Accuracy
in the Field
• When do you need the
accuracy?
– Navigation
– In-field verification of
position
• Real-time Differential GPS
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GPS requirements - Accuracy
• When do you need the
accuracy?
– Office based spatial
analysis
– Map making
• Post-processed
Differential GPS
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Where do you need to be?
12m
1m
1cm
1/2cm
2m
50 m
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GPS requirements – field ready
• All day battery
• Temperature range
– 50 degrees Celsius or greater
• Outdoor viewable screen
Depending on terrain
• Ingress Protection (IP)
– Water and Dust proofing
• Rugged and hard wearing
– Drop, shock and vibration proof
36
Summary
• When planning GPS data collection consider
– what accuracy you really need and when you
need it
– the day to day conditions such as heat and dust
and rough treatment of equipment
– the GIS field software that can be used on the
device
• GPS is an established and proven
technology used worldwide for GIS data
collection
– Allows better decision making at a lower cost than
traditional data acquisition techniques
37
Thank You
SPATIAL INNOVISION
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