C-DOM
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Return to Project Overview Page A NOTE ABOUT GPS COORDINATES AND GPS INBUILT COMPASS A fundamental part of our redrawing process for the new Corregidor Maps with 5 ft contours is the collection of a series of GPS co-ordinates and matching them to the original 1932/1936 Maps. I'm particularly interested in those at old structures such as buildings and batteries that I can confirm on the old maps. We are using the WGS84 coordinate system and UTM projection. GPS only really use WGS84 and convert positions to other coordinate systems mathematically within the GPS itself. Projections available in this system include Geographic Projection and Universal Traverse Mercator Projection. The former uses degrees and decimal degrees; or degrees minutes and decimal minutes; or degree, minutes and seconds, as latitude and longitude. The latter uses metres east and north; and places coordinates within zones. The zone for the Philippines is 51P. What I can say from the outset about mapping, the earth, coordinate systems and GPS is that they are a compromise. Any map distorts the earth's surface to get it to fit on the flat map. Coordinate systems are distorted based on what spheroid they use i.e. mean of elevations and depressions of the earth's surface; or the mean distance they use from the centre of the earth to the earth's surface. Projections are distorted as they might be conical or cylindrical (most common, but there are others). Hence bearings, distances,shapes and areas can be distorted.
Most maps use the UTM projection, which gives
the least distortion of curved earth surface when converted to a flat map.
Therefore, maps produced using the UTM projection display shapes and angles
undistorted. However, UTM maps contain some
scaling particularly the further from the equator one travels north or
south. What is positive about GPS, maps and the Philippines is that
the country is near the equator and therefore distortion is less than for,
say Japan or Canada. Corregidor is also a small island and again distortion
would be minimal. So, Corregidor should have everything going for it
regarding GPS and maps
GPS units contain a compass that ascertains bearings from consecutive satellite readings when the unit is moving. However, if the unit is not moving it acts like a magnetic compass. In the event that you want to use the compass on a daily basis, it should be calibrated daily. It will also need recalibrating when batteries are changed or if moving to distant locations. Double check calibration by turning the unit on; wait for satellites; press page key until the compass card is visible; press menu key once; scroll down to calibrate compass; and press entr key. Start will highlight, press entr key and rotate two full circles in the same spot. The GPS will tell you when to stop and if you are rotating too fast or slow, probably just as well because you are probably starting to feel dizzy by now. Once you get the calibrated message, press the quit key and you are done. I'm a bit doubtful of the value of the compass for navigation as it is probably only at it's most accurate when you are moving. Stationary - mine take ages to point north and does not return to north if moved - it can be 10 degrees either way. I put it on our concrete floor to let it settle down and it went absolutely crazy. I think this was because of the steel mesh in the concrete floor. So I'm also wondering how thick the layer of bomb and shell fragments is in the soil of Corregidor. I'll bring a decent compass for January 2009. Martyn Keen
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* New Co-ordinates are marked "n" and corrected ones are marked "c" |
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Coordinates courtesy of Steve Kwiecinski |
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