GPS - terminolgy and functionality

Waypoint

A waypoint is nothing more than a fancy name for a spot. Usually, it refers to the intended destination. However, where you are standing could be a waypoint, so could a reef the other side of Tonga. Many handheld units refer to them as ‘landmarks’. They are the same things.

Latitude and longitude

For navigation purposes the sphere of the Earth is divided up by lines extending from one pole to the other. These lines are intersected by other lines that run parallel to the equator. The long ones, those that extend from the poles, are called longitude lines; the horizontal ones are known as latitude. The important thing about latitude is that the lines are an equal distance apart. Because they’re a known constant, they are essential navigation tools. As you know, a circle is broken down into degrees and minutes. Note very carefully here and now that it is very unusual to use seconds for navigation, rather the minutes are broken down into three decimal places. The spacing of these latitude lines form the basis of the nautical mile. One minute of latitude is one nautical mile. A nautical mile is about 6000 feet, so you can deduce that the decimal places on the latitude position are worth 600, 60 and 6 feet respectively. Because of this you can easily and accurately measure distance off a chart. Don’t, however, try this off the longitude lines. Remember that they converge at the poles and are at their widest divergence at the equator meaning the distance varies at every point of the chart. Our waypoints will have a unique designation, being the intersection point of the respective latitude and longitude. For us here in NZ south and east will precede them, as we are south of the equator and east of the dateline. So if you took the position off the chart for the dead centre of Mayor Island, you would end up with something like S 39.17.201, E 176.15.275.

Datum

The problem with our nautical charts is that they are a flat representation of a curved surface. Because of this and the way they are drawn, errors will occur. To makes things agree a reference point is required. This is called the datum. For nautical use the one we use is WGS 84. Now it gets a little tricky. All the new charts you see published will have a big purple ‘WGS 84’ sticker all over them. With these charts it is possible to plot a position directly off the chart and transfer it to the GPS. The other type of chart, and it’s more common, isn’t referenced to WGS 84. However, in the block of information that will be drawn in one of the corners will be a reference to the use of satellite navigation systems and WGS 84. Typically for the North Island’s east coast, it will tell you to alter the position by 0.1 or 0.11 of a minute southward. So, for our Mayor Island co-ordinates above, the corrected position is S 39.17.311, E 176.15.275. From the previous paragraph you will now know that the value of the correction is about 660 feet.

Selective Availability

This doesn’t mean the service is available only on Sundays, but is rather the fancy name given to the error factor that the Americans built into the system and randomly and constantly apply. The intention is to degrade our accuracy so we don’t disturb Bill and Monica with our nuclear warheads. I doubt that they are very worried. The error is described as 100m RMS meaning 90% of the fixes should be within 100m of the correct position. In reality, we tend to find the error fits into a range of 20-50 metres. It can be more, and I’ve seen it, and it can be less. We regularly hear stories — like the customer who lost his hapuku dropper, went to the exact mark and couldn’t find it. As he wondered what to do next he heard a beating noise on the hulls of the cat. It was his marker buoy just breaking the surface as the swells receded. The reality is, the error factor is much less than the ability of the average boatie to accurately anchor his vessel. It’s far more accurate than my pine tree over the red roofed house.

Editors note: (As at May 2000 the inherent 'error' in GPS was removed)