1. Position 1 CPF09 v1.4 Copyright © BSAC 2010 Chartwork and Position Fixing Position Fixing Methods

2. Chartwork and Position Fixing Position 2 CPF09 v1.4 Copyright © BSAC 2010 Outline Simple fix Position lines ●compass bearing ●transits ●uses of position lines Position fix Visual fix accuracy GPS Other methods

3. Chartwork and Position Fixing Position 3 CPF09 v1.4 Copyright © BSAC 2010 Simple fix ●close to you ●fixed & clearly identifiable ●shown on chart Ensure object is correct one Detour for simple fix often better than fix by other means Is GPS position a simple fix? X No - a lat/long is not clearly identifiable

4. Chartwork and Position Fixing Position 4 CPF09 v1.4 Copyright © BSAC 2010 Position lines Lines that can be identified at sea and on a chart Types ●compass bearing ●transits Spire bears 060°T Beacon bears 320°T Tower bears 090°T Beacon in line with spire Tower in line RH of church Beacon in line with headland

5. Chartwork and Position Fixing Position 5 CPF09 v1.4 Copyright © BSAC 2010 Transits Two identifiable objects in line with each other ●taken from a chart to use at sea ●recorded at sea for later use ●obtained from books or divers with local knowledge ●can be very accurate ±0.5° ●not visible at distance, at night, in fog or in mist

6. Chartwork and Position Fixing Position 6 CPF09 v1.4 Copyright © BSAC 2010 Choosing transits “Fast” ●objects wide apart ●near mark close to observer ●easy to see if lined up ●sensitive to sideways movement Suitable landmarks ●easily identified ●long lasting ●fixed to the spot ○headlands ○conspicuous buildings ○easily identifiable rocks etc ●avoid vegetation, buoys and tide dependent marks

7. Chartwork and Position Fixing Position 7 CPF09 v1.4 Copyright © BSAC 2010 Recording transits Conspicuous Permanent Precise Well-recorded ●sketch ●bearing ●context ●narrative

8. Chartwork and Position Fixing Position 8 CPF09 v1.4 Copyright © BSAC 2010 Uses of position lines Clearing line Leading line Bengore Hd Ballintoy Pt Sheep I Fix ●an intersection of 2 or more position lines

9. Chartwork and Position Fixing Position 9 CPF09 v1.4 Copyright © BSAC 2010 Fix Intersection of 2 or more position lines defines a point ●bearings ●transits ●a mixture of transits and bearings Exercise: ‘Mystery Point’ Lat.................... Long...................

10. Chartwork and Position Fixing Position 10 CPF09 v1.4 Copyright © BSAC 2010 Visual fix accuracy Errors ●all position lines have errors ●error effect increases with distance ●area of uncertainty Improving fix accuracy ●position lines at right angles ●use transits if possible ●extra position lines show area of uncertainty ●boat movement during fix What accuracy needed? ●clearing lines ●intermediate points ●ease of navigation Error in metres cocked hat

11. Chartwork and Position Fixing Position 11 CPF09 v1.4 Copyright © BSAC 2010 GPS Global Positioning System ●receiver determines position using signal from 4+ satellites ●augmentation systems EGNOS/WAAS ●GPS set displays lat/long ●calculates distance/bearing to any other lat/long 24/7 all-weather availability Accurate and easy to use Accuracy ●±15m and ±3-5m with EGNOS ●= good transits at 1nm and 400m Also easy to misuse!

12. Chartwork and Position Fixing Position 12 CPF09 v1.4 Copyright © BSAC 2010 GPS limitations Lat/long is just a number..... Geodetic datum ●Earth not an exact sphere ●different chart makers use different models ●significant differences Limitations ●must use correct datum ●garbage in garbage out ●large errors if satellites obscured ●relies on accurate chart ●can become inoperable ○electrical faults ○damage, theft ○military operations

13. Chartwork and Position Fixing Position 13 CPF09 v1.4 Copyright © BSAC 2010 Other methods Depth soundings ●checks other fix if deeper or shallower water nearby ●get-me-home in fog ●crude position line where depth changes sharply ●apply tidal height and transducer offset if depth variation is small Radar ●range and bearing of land ●height of aerial problem for small boats

14. Chartwork and Position Fixing Position 14 CPF09 v1.4 Copyright © BSAC 2010 Summary Simple fix Position lines ●compass bearing ●transits ●uses of position lines Position fix Visual fix accuracy GPS Other methods