3/31/2023 0 Comments Datum vs.dataIt is not necessarily the same angle made by "straight up," because the latter is distorted by gravitational variation over the earth. (Geodetic) latitude is the angle made by a vertical line to the horizontal. Moving in the corresponding direction on the spheroid for the same distance determines where the new point should go on the spheroid. The base point and direction allow surveyors to determine the distance and angle of any other point on the earth. A datum is usually given by a starting point and direction: it specifies where a clearly identifiable point on earth (the base point) should appear on the spheroid and it shows where a base direction, such as north, points on the spheroid at the base point. "Unprojection" expands a feature on a map and plasters it back onto the spheroid.It, too, is a mathematical operation. Projecting is an operation that mathematically distorts and shrinks a portion of the spheroid onto flat paper. Georeferencing assigns locations (in three dimensions!) to points on a spheroid. It is an idealization that does not account for local changes in topography. The spheroid models the shape of the earth's surface. Two things must happen when you draw a map: features in the real world must be "georeferenced" to a spheroid and the spheroid must be projected onto the paper. The following is excerpted from that article. Same point referenced to NAD_1983_CSRS 19.048248 26.666876 Decimal DegreesĪfter struggling with this question ten years ago, and finding many confusing things written about the topic, I published a brief article in Directions Magazine that presented an answer as simply, plainly, and accurately as I could make it. Similarly, it is possible to convert data from one datum to another without projecting it, as with the NGS's NADCON utility, which can shift coordinates from NAD27 to NAD83.Įxample of a point's coordinates referenced to different datumsĬoordinates referenced to NAD_1927_CGQ77 19.048667 26.666038 Decimal Degrees The transformations from geographic to projected coordinates are the same, but as the geographic coordinates are different depending on the datum, the resulting projected coordinates will also be different.Īlso, projecting data may result in a datum conversion as well, for example, projecting NAD_1927 data to Web Mercator will require a datum shift to WGS 84. For example, the State Plane coordinate systems can be referenced to either NAD83 and NAD27 datums. It is possible, and even common for datasets to be in the same projection, but be referenced to different datums, and therefore have different coordinate values. The datum is a integral part of the projection, as projected coordinated systems are based on geographic coordinates, which are in turn referenced to a datum. If you see data in a geographic coordinate system, such as GCS_North_American_1927, it is unprojected and is in Lat/Long, and in this case, referenced to the NAD 1927 datum.Ī Projection is a series of transformations which convert the location of points on a curved surface(the reference surface or datum) to locations on flat plane (ie transforms coordinates from one coordinate reference system to another). An example of a datum is NAD 1927, described below Ellipsoid Semimajor axis† Semiminor axis† Inverse flattening††Ĭlarke 1866 6378206.4 m 6356583.8 m 294.978698214Īll coordinates are referenced to a datum (even if it is unknown). A datum typically defines the surface (ex radius for a sphere, major axis and minor axis or inverse flattening for an ellipsoid) and the position of the surface relative to the center of the earth. Geographic coordinate systems (lat/long) are based on a spheroidal (either truly spherical or ellipsoidal) surface that approximates the surface of the earth.
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