NAD 27 - NAD 83 conversion
UTM co-ordinates (often called GPS co-ordinates by the general public) are used to define locations on the surface of the Earth. UTM co-ordinates consists of three elements: Zone, Easting and Northing. All locations on the Earth (more precisely, all locations between latitude 80 South and latitude 84 North) can be identified by knowing the zone, the Easting and the Northing. (You also need to know the hemisphere:Northern or Southern. In this discussion I will assume Northern).
But, there is an additional essential piece of information that is also needed: the map datum. Without getting too technical, think of a datum as a mathematical model of the size and shape of the Earth, or of a portion of the Earth. In 1927 the knowledge of the size and shape of the Earth (geodesy) was based on land surveys. Surveyors could not see across large bodies of water and thus could not measure these distances. Based on the measurements that could be done at that time, the North American Datum 1927 (NAD27) was established.
Decades later, with the advent of the GPS satellites, it was possible to get a better understanding of the size and shape of the Earth than ever before. These new measurements showed that the previous understanding was flawed. Based on the new understanding, a new datum, North American Datum 1983 (NAD83) was created.
The consequence of the new datum is that there was a shift in both UTM grid co-ordinates and in geographic co-ordinates (latitude and longitude). All Canadian topographic maps created since 1990 are in the new datum (NAD83). Earlier maps are in NAD27. When using a GPS and referring to a map it is important that the GPS be configured to the same datum as the map.
A consequence of shifting between the two datums is a shift in co-ordinates. The shift is not consistent in all places. The shift is a 'rubber sheet', where the magnitude and direction of the shift varies from place to place.
You can use a GPS to display a shift in co-ordinates when you change between NAD27 and NAD83. But is the shift displayed by the GPS accurate? The following discussion will show some limitation in the shift shown in the GPS. There are only certain datum options displayed in the GPS (e.g. NAD27 Alaska and NAD27 Canada). NAD27 Yukon is not an option. Note: Although setting a GPS to either NAD27 Canada or NAD 27 Alaska will create errors in co-ordinates, the error is greater using NAD 27 Alaska. Thus we will only look at the error in NAD 27 Canada. If you wish to use a GPS in conjunction with a NAD 27 topographic map (in Yukon), it is better to configure the GPS to NAD27 Canada.
We will look at a position shown in UTM (NAD83) and see what the published NAD27 co-ordinates are using various techniques. The location I'm analyzing (waypoint 1) is just outside my office window (8V 494700 E, 6735000 N, NAD83).
The datum in the mapping software is now changed to NAD 27 Canada. Note: if I had this waypoint stored in a GPS and shifted the datum from NAD83 to NAD27 Canada I would get the same new results, (8V 494819 E, 6734816 N, NAD27).
Note that there has been a change in Eastings of +119 m, and a change in Northings of -184 m. But are the NAD 27 co-ordinates accurate? Is the change in Eastings and Northings correct? To test this we can go to the following website maintained by the Geodetic Survey of Canada. The results from this site are considered as good as it gets.
In this case we want to convert UTM co-ordinates from NAD83 to NAD27. Follow the links, fill in the fields and click on the calculate transformation button.
The results are intended for precise (survey quality) data. Thus they are published to the mm. With a simple handheld GPS that level of precision is unwarranted. Here are the precise results.
The actual shift (rounded to the nearest metre) in Eastings is +102 m and in Northings is -172 m. Thus the true NAD 27 co-ordinates for the waypoint are 494802 E, 6734828 N, not 494819 E, 6734816 N, as shown on the GPS. The discrepancy between the two results is 17 m in Eastings and 12 m in Northings. These are the sides of a right angle triangle. The hypoteneuse, the total error, is about 21 m.
So, what! and how can you get around this problem?
If you collect GPS data carefully (in other words wait until there is a reasonable accuracy before marking a waypoint) and if you transfer data electronically using a data cable and a download program such as DNRGarmin or Oziexplorer, your position will be good-within the accuracy limitations of the GPS, no matter what datum your GPS was configured in. But, to see accurate co-ordinates be sure to display the map in NAD83.
If you wish to manually enter NAD 83 co-ordinates into a GPS be sure the GPS is configured to NAD83 and then carefully enter the numbers.
If you have a table of NAD27 co-ordinates and wish to enter these into the GPS it is a bit trickier. If you simply configure the GPS to NAD 27 Canada and enter the numbers you are creating a systematic error of about 21 m. To avoid this you will need to take an extra step. You will need to, firstly, convert the NAD 27 co-ordinates to NAD 83 using the NTv2 program shown above. Then, with the GPS configured to NAD 83, you can manually enter the co-ordinates generated by the NTv2 program. There will still be a certain amount of random GPS error, likely in the 5-10 m range. But, by using the NTv2 program, to convert from NAD27 to NAD83, and relying on the NAD 83 co-ordinates, you will eliminate the 20 m+ systematic error inherent in the more simplistic conversion used by the GPS.
A practical case study- finding a legal survey post given NAD 27 co-ordinates
Older survey plans often display NAD 27 UTM co-ordinates. Do not configure the GPS to NAD 27 Canada and enter these numbers. Why? Because the NAD 27-NAD 83 conversion in the GPS is not accurate, at least not here in the Yukon. Instead, use the Geodetic Survey's conversion program to determine the correct NAD 83 co-ordinates, configure the GPS to NAD 83 and enter the NAD83 co-ordinates.
The "output" co-ordinates were entered as a waypoint into the GPS, with it configured to NAD 83. The waypoint was labelled NAD83. To compare, a second waypoint, labelled NAD27 was created by entering the "input" co-ordinates into the GPS, with it configured to NAD27 Canada. Then the GPS was used to search for the monument.
When searching for waypoint NAD83, the monument was easily found. Standing over the monument the GPS error was 1-2 m.
However, when searching for waypoint NAD 27, I was sent about 21 m away, and thus would not have found the monument if I searched within a 5-10 m radius. (see the following map). Thus, always configure the GPS to NAD83 and input NAD 83 co-ordinates into a GPS, if you wish to avoid the 20 m conversion blunder. If you have NAD 27 co-ordinates of course you need to use the NTv2 conversion first to calculate the correct NAD 83 co-ordinates.
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Last update: Friday, May 29, 2009 at 10:46:30 AM
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