How often do you find that existing records are not mapped correctly or accurately enough? Or, how many times do you ask yourself these questions: Which asset is it? Why is the map wrong? How can I find it accurately if its buried? The same old adage “rubbish in –rubbish out” applies to existing or legacy data when performing inspections and data maintenance projects. Understanding issues such as GNSS (GPS) and map error can help reconcile discrepancies and drastically improve data quality, with some basic consideration of what’s happening.

1. Improving the Accuracy ofImproving the Accuracy of
field data cost-effectivelyfield data cost-effectively
, Trimble Navigation Limited

2. AgendaAgenda
• Welcome and Introduction
• Accuracy Considerations
• Solution considerations
• Cost considerations
• Summary

3. How often do you find that existing records
are not mapped correctly or accurately
enough?
IntroductionIntroduction
• Which asset is it?
• How can I find it accurately if its buried?
• Why is the map wrong?

4. Which asset is it?
IntroductionIntroduction

5. 10cm
60cm 3m
1m
>5m
How do I find it accurately if its buried?......

6. Why is the map wrong?Why is the map wrong?
Historically, Ordnance Survey maps were produced at
different scales using different surveying methods
resulting in varying degrees of accuracy.
There are four accuracy categories.
•1:1250 scale
•1:2500 scale resurvey/reformed
•1:2500 scale overhaul. Maps originally
recompiled from pre-1946 County Series mapping
•1:10 000 scale.
Ordnance Survey describes accuracy in two ways.
•Absolute or positional accuracy.
•Relative accuracy
Source – www.gov.uk

7. Accuracy ConsiderationsAccuracy Considerations
Absolute accuracy – A measure which
indicates how closely the coordinates of a
point on a map eg. Ordnance Survey, agrees
with the true National Grid coordinates of the
same point on the ground eg. captured with
GNSS.
Relative accuracy – Relative accuracy
compares the precise distance between
features measured in the real world to the
distance between the equivalent features in
the data.

8. Accuracy ConsiderationsAccuracy Considerations
Source – Ordnance Survey

9. Source – www.gov.uk
Absolute
accuracy
Scale RMSE* 95% confidence
level
99% confidence
level
1:1250 (urban) <±0.5m <±0.8m <±0.9m
1:2500 (rural)
(resurvey or
reformed)
<±1.1m <±1.9m <±2.4m
1:2500 (rural) <±2.8m <±4.7m <±5.8m
1:10 000
(mountain and
moorland)
<±4.1m <±7.1m <±8.8m

10. Source – www.gov.uk
Relative
accuracy
Scale Relative error 95% confidence
limit
99% confidence
limit
Maximum
measured
distance
1:1250 (urban) <±0.5m <±0.9m <±1.1m 60.0m
1:2500 resurvey
or reformed
(urban and
rural)
<±1.0m <±1.9m <±2.5m 100.0m
1:2500 overhaul
(urban and
rural)
<±1.8m <±3.6m <±4.7m 200.0m
1:10 000
(mountain and
moorland)
<±4.0m <±7.7m <±10.1m 500.0m

11. What is relative and absolute position?What is relative and absolute position?
Source – Ordnance Survey

12. Working with Map ErrorsWorking with Map Errors
• Always work to a minimum of map accuracy
• Network assets are predominantly mapped
• Relatively in urban areas
• Absolutely in rural areas
• Always need to be correctly shown on
the map
• High-accuracy GNSS for new data capture
• Store absolute coordinates and capture
method. Important for:
• Modelling
• Relocation
• Quality

13. Achieving Map AccuracyAchieving Map Accuracy
• Choose the right tool for the job
• Ensure Mobile GIS software is fit for
purpose
• Use up to date OS transformations
OSTN02/OSGM02
• Simple “plug and play” scalable GNSS
• Create quality flags in workflow/database
• Drive behaviour of field teams to capture
data consistently

14. Technology ConsiderationsTechnology Considerations
• GNSS (not just GPS)
• Capturing GNSS quality flags
• If not then manual methods need to be
robust (minimum 2 dimensions)
• Integrating other sensors (eg. laser
rangefinder, Cable locator)
• Real-time corrections
• Post-processing GNSS

15. Real-time Correction ServicesReal-time Correction Services
Satellite- Based Augmentation
Systems
Free service
Sub-metre accuracy
achievable
Limited

16. Real-time Correction ServicesReal-time Correction Services
Network VRS (Trimble, Leica, TopCon)
Satellite corrections eg. RTX, Atlas
Usually require additional antenna/firmware option
Very accurate GNSS positions, typically 30cm-1cm
Convenience, no need to setup an additional base
 Subscription Costs (monthly to annual PAYG)

17. Solution ConsiderationsSolution Considerations
Bluetoooth
GPS/GNSS
Tablet/Smartphone
+ Bluetooth GNSS
Spike
Laser Rangefinder +
SmartPhone
attachment =
Calibrated Photos
Survey Grade GNSS
eg. Spectra Precision
SP80/SP60
eg. Arrow series
eg Trimble R1
GNSS-enabled
eg. DSLR
camera or
Cable locator

18. Solution ConsiderationsSolution Considerations
Cloud
Custom
Reports
HTML
Print to PDF
COTS or
Custom
Data Collection
Asset Inspection
eg. “BYOD” + GNSS
+ other sensors Data
Engine
Web-GIS/
Dashboard

19. Cost ConsiderationsCost Considerations
£300
£1,000
£2,500
£8,500
£300
£500
£750
£1,900
£5,000
£7,000
£0
£1,000
£2,000
£3,000
£4,000
£5,000
£6,000
£7,000
£8,000
£9,000
Consumer Prosumer Mapping Surveying
Cost£
Accuracy v Cost
GNSS
Software
All in one
Subscriptions

20. 10cm
60cm 3m
1m
>5m
How accurate do you need to be?......

21. About UsAbout Us
MGISS help organisations choose the right
mobile GIS solution by:
•understanding your requirements
•providing independent advice
•considering the most cost-effective approach
for you
•reducing your risk when building a business
case and
•supporting throughout the project lifecycle

22. Decisions…DecisionsDecisions…Decisions
MGISS are here to provide:
•Independent Consultancy/Advice
•Sales, Service, Training and Support
•Discovery days/ More Webinars!

23. www.mgiss.co.uk │ @mgissuk
mike@mgiss.co.uk
sales@mgiss.co.uk
T +44(0)151 329 0820│M +44(0)7841 070286
IC1, Liverpool Science Park, 131 Mount
Pleasant, Liverpool, L3 5TF