HOW ARE LATTITUDE AND LONGITUDE PROJECTED ?

Imagine a basketball as the earth. If you were to put a light bulb at its center and place a flat sheet of paper touching the edge of the ball when the light was switched on the lines of the basketball would be projected on to the paper, representing latitude and longitude. This is a basic mathematical projection which we have accepted as being NAD27.

Now we know, because of more accurate satellite measurements, the earth is not a true spheroid. It is a little squashed at the poles, therefore the mathematical center of the earth has moved. Now the same latitude and longitude that are projected have moved, so now the same physical point of the earth has a new mathematical value which we call NAD83.

The ideal solution of all of the above problems would be a spheroidal model that has both the correct equatorial and polar radii, and is then centered on the actual center of the Earth. One would then have a spheroid, that when used as datum, would accurately map the entire Earth. All latitude / longitude on all maps would agree. That spheroid, derived from satellite measurements of the Earth is GRS80, and the datum's that match it are NAD83 and WGS84.

SOME NECESSARY FUNDAMENTALS

Maps use a system of latitude and longitude lines to specify location. Lines of latitude, or parallels, are evenly spaced circles running parallel to the equator. Parallels are identified as being a certain number of degrees north or south of the equator (figure 1). Lines of longitude, or meridians, are drawn so that they converge at the north and south poles of the globe
(figure 2). The meridian lines are also indicated in degrees, with the line that passes through Greenwich near London, England, labelled 0 degrees. Meridian lines to the east of this line are labelled in degrees west longitude, also up to 180 degrees. The 180 degree line, passing through the Fiji Islands in the Pacific, is the international date line.

NAD 27 / NAD 83 CANADA

The coordinate system at present used in Canada is linked with the Clarke 1866 spheroid and the associated datum NAD27 (North American Datum 27) which is used to generate Latitude/Longitude values that are converted into UTM co-ordinates. That is to say only Latitude/Longitude points surveyed using the NAD27 datum should be converted to X/Y's (UTM's) using the NAD27 (UTM) datum, and do not tie to Latitude/Longitude's referenced to any other datum (i.e. everywhere outside of North America). It is now possible, with satellite data, to create spheroid/datum that is valid throughout the entire world. That datum is WGS 84, (World Geodetic System 84) which is basically the same as GRS80 (Geodetic Reference System 80), which is for all intents and purposes the same as NAD83. It has been proposed that the NAD27 datum be replaced by the NAD83 datum. This is obviously a good thing, having all Latitude/Longitude co-ordinates agree with each other throughout the world.

WHY DIFFERENT A.T.S. VERSIONS ?

At the moment there are five different A.T.S. versions in Alberta: Prior 86, 2.2, 2.6, 3.1, 3.2 and two in Saskatchewan: STCF 1.0 and 2.5. It is best to think of each of these versions as an up-date. As time goes by and technology gets better we are finding errors in the original surveys. Not surprising when you consider the equipment and conditions under which the first surveys were conducted. Each time the government has what it thinks is enough changes, it will come out with a new version. Thankfully they are finding less and less errors. The differences between the A.T.S. versions are not to be confused with the differences between the NAD27 and NAD83. They are two totally different problems but any combination of any of these problems that are allowed to be loaded to a database will have serious repercussions.

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