Gathering and using spatial data requires a way to record the location of whatever is being observed (e.g., buildings) that preserves the spatial properties of the data (e.g., the distance between points in a dataset). Spatial reference systems are used to record the spatial properties of data in a meaningful and translatable way.
Spatial reference systems are typically made up of two components, a 2-dimensional coordinate system, which defines the rules of how points are assigned to observations of spatial data, and a 3-dimensional datum, which defines the origin and scale of the coordinate system. These two components in combination allow spatial phenomena to be meaningfully translated and recorded as data.
The coordinate system of a spatial reference system is used to define the location of a spatial object using a set of numbers.
Think about a simple cartesian coordinate system that you might have been exposed to in math class. Typically, an x-coordinate is used to represent the horizontal position of an object, and the y-coordinate is used to represent the vertical position. Combining these two coordinates describe the location of a point in the 2-dimensional plane.
Geographic coordinate systems work the same way, but for positions on the surface of the Earth.
For example, latitude and longitude area coordinate system where latitude represents the north/south location of a point, and longitude describes the east/west.
For analyzing spatial data in a relatively small region of interest, geographic coordinate systems can be accurate while approximating the shape of the Earth as a sphere. For an analysis that covers a larger area, the approximation of an ellipsoid (it has two radii instead of one) is much more accurate (+-0.3%). However, the Earth’s true shape is not an ellipsoid either. To maximize the accuracy of analysis where spatial data spans a large area of the Earth, variation across the Earth’s surface need to be considered.
A geodetic datum (also called a datum for short) defines a reference frame for spatial coordinates, measurements, and calculations. It consists of a selected ellipsoid and a definition of the position of the ellipsoid relative to the center of the geoid (the smooth but irregular surface of the Earth).Some of the more well-known datums include WGS84, NAD83, OSGB36.
If you have worked with spatial data before, you made have heard of projections. Projections are a special type of coordinate system that specifically account for the distortion that occurs when translating a 3-d object (the Earth) to a 2-d representation (e.g., a map). Depending on the projection chosen distortion will occur in one or more aspects of shape, area, distance, and direction. It is important to note that each map projection will preserve only or two of the four spatial properties.
A common map projection is the Mercator projection, though it has limitations.
The Mercator Projection was designed as a navigational tool for sailors. It preserves the shape of coastlines, and direction (rhumb lines, useful for navigation) while distorting area. Moving away from the Equator to the poles, the size of landmasses appears much bigger than they really are. For instance, Greenland owing to its closeness to the North Pole will appear roughly the same size as Africa.
This shows how important the choice and understanding of the projection can be to get accurate results.
How does Alteryx Designer handle Spatial Data?
All spatial data read into Alteryx is automatically transformed to a WGS84 datum with a latitude and longitude coordinate system.
Spatial calculations in Designer are performed on a sphere with a radius between the polar and equatorial radii of the Earth. This does result in some distortion in calculations. Near the equator, distance calculations can be 0.2% smaller than their actual size, and near the poles, distance calculations can be 0.2% larger. Within the US and Europe, distance calculations are more accurate. Alteryx does not use any projected coordinates when performing spatial calculations.
Projections can be specified when writing out data spatial formats via the Edit projection dialog box. This feature is supported in the following spatial file formats: MID/MIF TAB, SHP, Oracle, and ESRI Personal Geo-Database.
All maps from the Report Map tool are drawn in Spherical Mercator projection.