What map datum should i use with my gps

Latitude-Longitude: Most people, I'm assuming, are familiar with "Lat-Long" at least in name and can point to the latitude and longitude lines on a globe. Many of us learn a little about this system in school. But as a refresher, here are some basic points about this angular coordinate system:. This is because it's a grid system specifically designed for two-dimensional maps. Imagine a map of the world spread out on the table. On that map, there are horizontal and vertical lines, all intersecting at degree angles.

These intersecting lines create a grid rows of 60 boxes. The columns are labeled by number 1 through 60 and the rows are labeled C through X, omitting O and I, so they aren't confused with the numbers zero and one.

Each UTM "zone" is therefore expressed by a number and letter. For example, I live in Flagstaff, Arizona, which is in zone 12S. The "S" should not be confused with the word "south. Each zone is centered on longitude line. The lettered rows are based on latitude bands running every 8 degrees. Eastings are found along the top and bottom of topographic maps.

Northings are found along the sides of topographic maps. When looking for a UTM coordinate on a map, which you can then enter into your GPS, you'll see that the numbers along the map edges show just the first four digits--the millions, hundreds of thousands, tens of thousands and thousands of meters. For example, if the Easting is , you'll actually see just and not the zero in front of it. If the Northing is , you'll see just the You can make this measurement using plastic grid readers, which you can buy from the U.

Or you can make a UTM grid reader with a small piece of paper, marking both edges out from the corner in meter increments. You can use the map's bar scale as a guide. Here's a close-up example of what UTM zones look like. While there is some redundancy among these articles, I find that I learned something from each of them. And the more you read, the more these concepts sink in. Of course, then you have to get outside and practice with a map, compass and GPS in the field, because no amount of reading can take the place of hands-on experience.

Which coordinate system your GPS is set on is just one part of the equation. The next question is which datum it and your map will be using with that coordinate system.

Datums are necessary for map-making, where the three-dimensional landscape has to be converted to a two-dimensional surface. But there are many different datums, and not all maps use the same one. Maps have an information block, usually in the bottom-left corner. This where you can find out things like who made the map, when it was made, when it was edited, and so forth.

Aeronautical maps like those used by helicopters are made using the global WGS84 datum. Datums are also part of the equation on a GPS. You can change the datum at any time if necessary. It's also important to note that some units, such as the Garmin Extrex receivers, will revert to the WGS84 datum whenever you change coordinate formats ie. So if you're intending to use a datum other than WGS84, you'll have to reset it.

You MUST set the datum on your GPS to the same datum as the map you're using in order to get accurate position information for using on that map. The datum you have setup in your GPS receiver must match the datum used to create the map you are using.

The three common datums in use in the Continental United States are:. Some GPS units subdivide this datum into several datums spread over the continent. Map making begins with surveying. When you survey large areas, you need to take the curvature of the earth into account in your calculations. The earth is a lumpy bumpy three dimensional thing, that can be approximated with a nice clean mathematical ellipsoid. Various regions of the world selected an ellipsoid that best approximated their portion of the earth.

Prior to advent of satellites, most surveying was done on the ground or by using photos taken from an airplane. Early maps and surveys were carried out by teams of surveyors on the ground using transits and distance measuring "chains". Surveyors start with a handful of locations in "known" positions and use them to locate other features. These methods did not span continents well. Frequently they also did not cross political borders. The "known points", their positions and the ellipsoid used are the information that the map datum is based.

As space based surveying came into use, a standardized datum based on the center of the earth and an ellipsoid the was a good fit to the entire surface of the earth was developed. This means the coordinates in Australia are projected forward to the date of 1 January Since then:. These refinements to the reference frame and many of the local scale distortions had not been reflected in changes to the Australian datum since GDA GDA provides a more robust and accurate datum which is more closely aligned to global positioning systems like GNSS and will ensure that Australian industry, the research community and the public can accurately align themselves and their data.

This re-adjustment was not adopted by all States resulting in both versions being in use prior to adoption of GDA Users need to be very careful, as there remains a lot of data and information based on the old system and there is approximately metres between the two.

The AGD datum was designed to fit the shape of the Earth in the Australian region as closely as possible. Further, it was fixed in relation to one central point, Johnston Origin , and therefore was not affected by tectonic plate movement; the datum went with the plate. The major drawback of this datum was that it could not be rigorously related to international systems such as GPS.

The National Transformation Grid overcomes this problem to a very large extent by developing local parameters based on a national grid.

For this reason, the National Transformation Grid is recommended for such transformations. As we have pointed out, this is, strictly speaking, insufficient for a datum. The version of WGS84 is commonly not quoted nor is any reference epoch. For the purposes of charting and navigation however these considerations are generally insignificant. ITRF at Projections and datum should not be confused.

See the explanation about how these differ in Datums - The Basics. Heights are expressed in relation to a height datum. The Geoid To understand height datum it is preferable to first understand the concept of the geoid. The Geoid is an imaginary surface which coincides closely with mean sea-level over the ocean and its extension under the continents.

Because gravity is variable, depending upon things such as variable Earth mass, the geoid is an irregular surface.

Surveyors traditionally transferred heights between benchmarks using levelling equipment based on gravity. Benchmarks are stable marks placed on the ground which have known heights above sea level - they are sometimes called vertical control points.