Datums Projections Coordinate systems Mapping Exercise GPS Coordinate Systems. Outline

Transcription

1 Datums Projections Coordinate systems Mapping Exercise GPS Coordinate Systems Outline

2 Projections Projecting Our Globe Onto A Flat Surface Map projections are a representations of a sphere (the Earth) in two dimensions. Globes are the most accurate way to represent the surface of the Earth. However, because it is not practical to carry a globe into the field, map makers must figure out how to represent a round map on a flat piece of paper, or on a flat screen. Over the centuries many different ways of representing the round Earth on flat paper have been developed. Each of these methods is referred to as a map projection

3 Projections

4 Projections Orthographic Projection (Equatorial): Preserves the appearance of a globe and shows only half the Earth at a time. Distorts shape and direction and is useful for illustrations

5 Projections Planar (Azimuthal) Projection (Lambers Equal Area Azimuthal Projection): Used to represent the projection of a region (northern hemisphere) onto a plane tangent to the globe, in this example, the north pole is used.

6 Projections Conic Projection (Lamber Conformal Conic): Made by fitting a cone over part of the globe. The projection depicts mid-latitudes shapes and areas well. It is used to depict large regions up to the size of Canada

7 Projections Cylindrical Projection (Mercator): is based on a cylinder tangent to the equator. Good for equatorial regions but greatly distorted at high latitudes. This one of the oldest and most common projections

8 Projections Transverse Cylindrical Projection (Universal Transverse Mercator, UTM): is based on a cylinder tangent to the globe along a chosen pair of opposite meridians. The scale of the map is constant only along the central meridian. UTM is a commonly used projection for NTS maps ranging in scale from 1:24,000 to 1:250,000. The UTM projections are based on 60 UTM Zones each defined by a central meridian and covering 3 degrees of Longitude to the East and West.

9 Scale Large Scale Map vs Small Scale Map Large Scale A map which depicts a small territory is referred to as a large scale map. Ie 1: cm = 5000 cm A large scale map only shows a small area, but it shows it in great detail. Small Scale A map depicting a large area, such as an entire country is considered a small scale map. Ie 1:250, 000 In order to show the entire country the map must be scaled down until it is much smaller. A small scale map shows more territory, but it is less detailed.

10 Coordinate Systems A coordinate system is a means for identifying a point on the earth on a planimetric map. The coordinates systems is typically defined using an x- and y-ordinate or northing and easting A choice of systems to display your location Latitude/ Longitude - standard system of measurement used on most maps throughout the world Universal Transverse Mercator UTM newer Grid System gaining more acceptance

11 Coordinate Systems Lat Long The most commonly used coordinate system today is the latitude, longitude, and height system. The Prime Meridian and the Equator are the reference planes used to define latitude and longitude. If you are working with maps that cover more than 6 degrees of longitude or are 1:1,000,000 scale or less, you will probably want to use lat/lon coordinates.

12 Coordinate Systems Lat Long

13 Coordinate Systems Latitude/ Longitude - three common formats: DDD MM' SS.S Degrees, Minutes and Seconds 54 18' 23.1 N ' 52.5" W DDD MM.MMM Degrees and Decimal Minutes ' N ' W DDD.DDDDD Decimal Degrees N W

14 Coordinate Systems Degrees, Minutes and Seconds DDD MM' SS.S" 54 18' 23.1" N ' 52.5" W This is the most common format used to mark maps. It's also the most cumbersome to work with. It's a lot like telling time Degrees and Decimal Minutes DDD MM.MMM' ' N ' W This is the format most commonly used when working with electronic navigation equipment. Decimal Degrees DDD.DDDDD N W This is the format you'll find most computer based mapping systems displaying. The coordinates are stored internally in a floating point data type, and no additional work is required to print them as a floating point number. Which format should you use? First off, if you are working with other people who have agreed upon a format to use, then you should probably use that format. Next, you will want to look at the maps, lists of coordinates, and any software you may be using. If you can find a consistent format among them, your work will be easier. You can set your GPS to display any one of these three formats. Locations can be entered into the GPS with the selected format, and then by switching the display format setting, viewed in a different format. Decimal degrees less room for confusion ( my choice)

15 UTM Some of the advantages of UTM coordinates include... Square grids East-West units of measure are the same as North-South units No Negative numbers Decimal based, no fussing with minutes and seconds. Coordinates translate directly to distances on the ground. Coordinate precision is easily understood. No need to wonder what distance a tenth of a second of longitude represents. It's easy to abbreviate coordinates when working in a small area.

16 Universal Transverse Mercator UTM Gaining in popularity due to its ease of use UTM system divides the earth into 60 zones each 6 degrees of longitude wide. These zones define the reference point for UTM grid coordinates within the zone. UTM zones extend from a latitude of 80 S to 84 N. In the polar regions the Universal Polar Stereographic (UPS) grid system is used.tood. No need to wonder what distance a tenth of a second of longitude represents.

17 Example UTM 10U E N Eastings UTM easting coordinates are referenced to the center line of the zone known as the central meridian. The central meridian is assigned an easting value of 500,000 meters East. Since this 500,000m value is arbitrarily assigned, eastings are sometimes referred to as "false eastings" An easting of zero will never occur, since a 6 wide zone is never more than 674,000 meters wide. Minimum and maximum easting values are: 160,000 me and 834,000 me at the equator 465,000 me and 515,000 me at 84 N Northings UTM northing coordinates are measured relative to the equator. For locations north of the equator the equator is assigned the northing value of 0 meters North. To avoid negative numbers, locations south of the equator are made with the equator assigned a value of 10,000,000 meters North.

18 UTM

19 UTM

20 UTM Explained Example 10U E N

21 Datums Simply put, a datum is a model used for calculating the location of the Earth's center. This center calculation is used as a reference point in establishing coordinate locations, such as latitude and longitude, on the Earth's surface "datum" is the base reference for an X, Y coordinate system. Because the earth is not a perfect sphere, we developed mathematical models for the accuracy of coordinates in specific geographic areas. Known as "local datums," they use an irregular ellipsoid model to obtain a "best fit" of the earth's surface for a particular region We are lucky in that NAD83 and WGS 84 are essentially identical for our area of North America so they can be used interchangeably

22 Datums Common Datums NAD27 - Older maps 1927 North American Datum NAD 83 Newer maps 1983 North American Datum WGS World World Geodetic System Local Datums ED50: European Datum OGB36: Ordnance survey of Great Britain 1936 ADINDAN: Set of African datums

23 Datums North American Datum of 1927 (NAD27) is a datum based on the Clarke ellipsoid of The reference or base station is located at Meades Ranch in Kansas. There are over 50,000 surveying monuments throughout the US and these have served as starting points for more local surveying and mapping efforts. Use of this datum is gradually being replaced by the North American Datum of North American Datum of 1983 (NAD83) is an earth-centered datum based on the Geodetic Reference System of The size and shape of the earth was determined through measurements made by satellites and other sophisticated electronic equipment; the measurements accurately represent the earth to within two meters. There are differences in the two ellipsoids ranging from feet in the western US to several tens of feet in the central and eastern US as show below: NAD27 versus NAD83: the 'NAD shift' (North America) There can be a difference (NAD27-NAD83) of up to 170 metres in y (N-S) and 70 metres in x (E-W), corresponding to the difference between the two ellipsoids

24 Review A map projection is one of many methods used to represent the 3- dimensional surface of the earth or other round body on a 2- dimensional plane ( paper map or computer screen) in cartography (mapmaking). A datum is a model of the earth that is used in mapping. The datum consists of a series of numbers and reference points that define the shape and size of the ellipsoid and it's orientation in space. A datum is chosen to give the best possible fit to the true shape of the Earth A coordinate system is used to refer to a specific point on the Earth and its corresponding point on the projected map. Geographic Lat long UTM - Grid