Glossary
azimuth
An azimuth is the horizontal direction of a vector, normally measured in degrees clockwise from true north, as the degrees on a compass. Sometimes, the term "grid azimuth" is used for the angle on the map from grid north (up on the map) to the vector.
See also Crs.AngleFromAzimuth.
azimuthal projection
One family of projections are called azimuthal. An azimuthal map has a central point, and the great circles through that point are mapped to straight lines, which intersect with correct angles. That is to say: from the central point, the azimuth to another point can be measured directly on the map. In Carmenta Engine, the azimuthal projections are: AzimuthalEqualAreaProjection, AzimuthalEquidistantProjection, OrthographicProjection, PolarStereographicProjection, and StereographicProjection.
cell
The smallest part of a raster, containing a value that corresponds to a specific area on the ground. A raster cell is analogous to a screen pixel, but may correspond to a larger or a smaller area than a screen pixel does. Of course, if the raster is ideally fitted to the presentation scale and the screen resolution, then the areas of a raster cell and a screen pixel will be the same.
In some texts, for example the specification of the DTED format, the word cell is used in a different sense, for something we call tile.
color raster
A raster that contains colors; in other words, an image. In Carmenta Engine, a color raster may be an indexed raster (1), or an RGB raster or an RGBA raster.
color table
A mapping from integers to colors, used by an indexed raster (1). In Carmenta Engine, a color table is normally a part of a RasterVisualizer, which can be attached to the Feature by a VisualizationOperator or by a DataSet.
configuration lock
A lock that must be used to synchronize access to objects in a configuration that has been loaded into memory. Please see Threading Model, configuration for more information.
conformal projection
One family of projections are called conformal. At each point in a conformal map, the local scale is the same in all directions. This is the same as saying that the shapes of small geographic objects are preserved: for example, the angle between two short lines is preserved. In fact, the angle between long lines is also preserved, but note that a great-circle segment on the earth is usually mapped to a curved line. Conformality is a very useful property for maps, and almost all large- and medium-scale maps use some conformal projection. For continental or world maps, however, the scale variation of a conformal projection becomes large, and other kinds of projections are often better. In Carmenta Engine, the conformal projections are: MercatorProjection, TransverseMercatorProjection, RosenmundObliqueMercatorProjection, LabordeObliqueMercatorProjection, HotineObliqueMercatorProjection, LambertConformalConicProjection, StereographicProjection, PolarStereographicProjection, KrovakNorthOrientedProjection, and NewZealandMapGridProjection.
conic projection
A conic projection is constructed by wrapping a cone around the earth. If the cone axis coincides with the earth axis, the parallel circles of the earth will be mapped to concentric circle arcs, while the meridians will be mapped to straight lines radiating from a pole. There are many kinds of conic projections. Carmenta Engine supports the AlbersEqualAreaConicProjection, the LambertConformalConicProjection, and an oblique variant of Lambert: the KrovakNorthOrientedProjection.
continuous raster
A singleband raster that contains numeric values of a continuous entity, like temperature or ground elevation. See also color raster and discrete raster.
convex hull
The smallest convex polygon that covers a set of points or geometries.
custom object
Custom objects are customer developed datasets, operators, symbols or visualizers that can add customer specific functionality to Carmenta Engine. For example, a custom dataset can be used to load geodata in a format that Carmenta Engine otherwise would not support.
A custom object is used in a Carmenta Engine map configuration by adding a CustomDataSetProxy, CustomOperatorProxy, CustomSymbolProxy or CustomVisualizerProxy object to the configuration.
A number of sample custom objects are included in the Carmenta Engine SDK. Please see An Overview of the Carmenta Engine Custom Object Samples for more information.
cylindrical projection
A cylindrical projection is constructed by wrapping a cylinder around the earth. In the equatorial aspect, the cylinder axis coincides with the earth axis. This means that the parallel circles of the earth will be mapped to parallel horizontal straight lines, not necessarily equidistant, while the meridians are mapped to parallel vertical lines that are equidistant. In Carmenta Engine, the cylindrical projections in equatorial aspect are: Projection.LongLat, EquidistantCylindricalProjection, EquidistantCylindricalAngularProjection, MercatorProjection, MillerCylindricalProjection, and DynamicLocalProjection. In the transverse aspect, there are the TransverseMercatorProjection and the CassiniSoldnerProjection. And in the oblique aspect, there are the RosenmundObliqueMercatorProjection, the LabordeObliqueMercatorProjection, and the HotineObliqueMercatorProjection.
dataset lock
discrete raster
Synonym: thematic raster. A singleband raster that contains integer values that represent a finite enumeration, like terrain types: WATER, FOREST, CITY, ...
edge effect
Many raster operations; for example, IsolineOperator, ShadeOperator and VerticalProfileOperator; can be performed only in the interior of a raster. This is due to an edge effect, which invalidates the result near the edges of the raster. The invalid result is usually discarded, so that a correct output raster will be smaller than the input raster. The difference, measured in number of raster cells, is the size of the edge effect. To hide the edge effects from a map user, raster tiles must be connected before they are processed, but this is handled automatically by the built-in raster operators. However, a custom operator for rasters may need to retrieve input via a RasterMergeOperator.
ellipsoid
In mathematics, an ellipsoid can have three axes of different length, but this is not needed for mapping. In geodesy, an ellipsoid is always an oblate ellipsoid of rotation (a spheroid), which is generated by rotating an ellipse around its minor axis. Such an ellipsoid gives a good approximation to the shape of the earth, and is used as the base for large- to medium-scale maps. (For continental or world maps, the earth can be approximated by a sphere.) See the Ellipsoid class.
format
A convention for storing geographic data on disk.
geodesic
A geodesic is another name for a great circle.
global configuration lock
See configuration lock.
graticule
A set of intersecting lines on a map, representing meridians and parallel circles. A graticule is a special case of a grid 1, since a graticule always consists of lines of constant longitude or latitude (rather than lines of constant easting or northing). In the Carmenta Engine documentation, we often use the term "LongLat grid" instead of graticule.
great circle
A great circle on a sphere is formed by a plane that intersects the sphere through its center, dividing it into two equal hemispheres. A segment of a great circle is the shortest route on the sphere surface between its end points (if the segment is less than 180°). On an ellipsoid, a great circle usually means a shortest route on the ellipsoid surface (a geodetic route).
grid
In geography, the term "grid" has a concrete meaning (1) and an abstract one (2).
A set of intersecting lines on a map, associated with a Crs. Each vertical line has a constant x coordinate (easting), and each horizontal line has a constant y coordinate (northing). If the Crs is not projected, but uses longitude and latitude, then the grid may also be called a graticule.
Another name for a (Crs). For example, "Swedish Grid" is an English name for the traditional Swedish coordinate reference system, also known as "RT90 2.5 gon V".
heterogeneous
A set of rasters are heterogeneous if they differ in important aspects, such as their Crs, cell size, color table, or undefined value. Opposite: homogeneous.
homogeneous
A set of rasters are homogeneous if each raster has the same Crs, the same cell size, the same color table and the same undefined value. The tile size need not be the same. Opposite: heterogeneous.
indexed raster
A singleband color raster where each cell value is an index into a color table.
A raster for which a spatial index has been generated.
insertion point
The insertion point is the position where texts or objects are inserted into a certain space. The term insertion point is also used for a RasterSymbol as a synonym for hot spot. Each RasterSymbol has its own hot spot within the symbol. This point defines what part of the symbol that will placed on the insertion point in the space chosen. For a RasterSymbol, the hot spot originally is the upper left corner, but it can be altered by using the properties RasterSymbol.XHot and RasterSymbol.YHot.
loxodrome
Loxodrome is another name for a rhumb line.
multiband raster
This term is typically used for satellite images that contain information about many wavelengths: for example, ultraviolet, green, and infrared. Each band of the raster contains the intensity of one wavelength. Once a multiband raster has been read by Carmenta Engine, it will be stored internally as either RGB (red/green/blue) or RGBA (red/green/blue/alpha), but some raster datasets can read other color models like CMYK (cyan/magenta/yellow/black) and convert them to RGB or RGBA. More generally, the bands do not have to represent wavelength intensities, and can represent things like temperature, humidity and air pressure. In such cases, you can choose to read just a single band by specifying HdrDataSet.Band or GdalQuery.Band.
oblique Mercator projections
An ordinary MercatorProjection is constructed on a standing cylinder and is best along the equator. If you rotate the Earth inside the cylinder before projecting, you can get a projection that is best along an arbitrary great circle instead. A rotation of 90 degrees gives a TransverseMercatorProjection, which is best along two opposite meridians. Other rotation angles give an oblique variant of Mercator, which is good for a region that is long, narrow and oblique.
As long as the Earth can be treated as a sphere (for world maps etc.), there is only one way to construct an oblique Mercator. But when the Earth must be treated as an ellipsoid, there is no perfect solution: there are no formulas that can achieve both conformality and constant scale along a geodetic route. So, one must compromise in some way. In fact, there are several alternative definitions of oblique Mercator projections, which are slightly different.
Rosenmund Oblique Mercator: developed in 1903 for Switzerland, and later used for Hungary. See RosenmundObliqueMercatorProjection.
Laborde Oblique Mercator: developed in 1928 for Madagascar, and used nowhere else. See LabordeObliqueMercatorProjection.
Hotine Oblique Mercator: developed in 1946-47 for East and West Malaysia, and later used for Liberia, Michigan, southeast Alaska, the Great Lakes and Denmark. See HotineObliqueMercatorProjection.
Cole defined an oblique Mercator for Italy, but it is not used by Italian mapping agencies. (J. H. Cole, The use of the conformal sphere for the construction of map projections. Survey of Egypt paper 46, 1943.)
Yet another oblique Mercator has been defined, where the conventional Transverse Mercator and the ordinary Mercator can be constructed by the general formulas, but the oblique aspect does not seem to agree with any previous oblique Mercator. (J. Engels and E. Grafarend. The oblique Mercator of the ellipsoid of revolution E²a,b. Journal of Geodesy 70(1-2), Nov. 1995.)
The Hotine formulas achieve the smallest deviations, and would be the preferred choice today.
offset position
The relative position of a text or a label is decided with a starting point at the offset position. This means that the user chooses what part of the text that should be located on the offset position. An X-alignment set to #right will place the end of the text on the position mentioned, which means that the text will be placed on the left hand side.
orthodrome
An orthodrome is another name for a great circle.
overview
Raster data in a fixed resolution is appropriate only for a limited range of presentation scales. To display the data efficiently in more zoomed-out scales, we need alternative versions of the data in lower resolutions; these are known as overviews. Most Carmenta Engine raster datasets can generate such low-resolution overviews for raster formats that do not natively support them. See for instance ImageDataSet.RasterCachePolicy for more information. For other raster formats, such as EcwDataSet and Jpeg2000DataSet, the same data file can contain both original data and overviews.
raster attribute table
A thematic raster dataset can have a raster attribute table, which associates each integer raster value with a set of attributes. For example, the raster value 40 could be associated with an attribute Land_Cover with the value "Forest" and an attribute Estimated_Height with the value 25.0, etc.
In Carmenta Engine, the ImageDataSet and the GdalDataSet can read raster attribute tables, which will be accessible as components of the DataSetInfo for the dataset. The tables can also be used implicitly from an ImageQuery or an GdalQuery for the dataset. See also Working with Raster Data.
raster, undefined value
A value associated with a raster and assigned to cells that have no defined value.
resolution
Tells how fine-grained a raster is. Small cells correspond to high resolution. A low-resolution raster is coarse; a high-resolution raster is fine.
RGB raster
A multiband raster where each cell explicitly contains a color coded as its red, green, and blue components. In Carmenta Engine, an RGB raster can be created programmatically in an application by creating a feature with a raster geometry or it can be generated when reading certain raster formats, for example those that can be read with ImageDataSet.
RGBA raster
A multiband raster where each cell explicitly contains a color coded as its red, green, and blue components plus an additional alpha component that gives the degree of transparency (lower values makes the color more transparent). In Carmenta Engine, an RGBA raster can be created programmatically in an application by creating a feature with a raster geometry or it can be generated by a RasterizeOperator or a LayerRasterizeOperator.
rhumb line
A rhumb line, also called a loxodrome, is a curve with a constant bearing or azimuth: that is, it intersects each meridian with the same angle. It is not the shortest route on the ellipsoid, except if it is a meridian or the equator.
With older navigation systems, it was easier to split up a route into smaller rhumb-line segments. Modern systems can handle the continuously changing bearing of the great circle great-circle route, but there is still a strong tradition of using rhumb lines in navigation to define waypoints etc.
A rhumb line looks straight on a MercatorProjection map. When an oblique rhumb line approaches a pole, it becomes a spiral that goes around the pole an infinite number of revolutions, but its length to the pole is still finite. An oblique rhumb line has no well-defined direction at the pole.
scale, map
The ratio between a measurement on a map and the true distance on Earth. For example, a map scale of 1 : 1000 means that 1 millimeter on the map is 1000 millimeters on Earth. The colon is a symbol for division, so a scale of 1 : 1000 could in theory be written as 0.001 instead. This explains why the scale 1 : 1000 usually is called a larger scale than 1 : 1000000 (because 0.001 is a larger number than 0.000001). In Carmenta Engine, though, the denominator (1000 or 1000000 in our examples) is often called the scale. For example, the View.Scale, as well as the Layer.MinScale and the Layer.MaxScale , refers to the denominator.
A plane map of the Earth cannot have constant scale everywhere (only a globe can have that). So nominal scale must be distinguished from local scale:
scale, local
For each very short line on a map, say a millimeter, there is a local scale that tells how long that line would be on Earth. The local scale varies with the position of the line. But it can also vary with the direction of the line: if it does not, then the map projection is said to be conformal. See also nominal scale and scale factor.
scale, nominal
The map scale that is printed in the margin of a map. It is called nominal because it is not true on the entire map. (That is impossible for a flat map of the round Earth.) The nominal map scale can be true on a central point, or on a central line, or on two parallel lines, or on a circle or some other curve. For example, on a MercatorProjection map, Greenland is drawn in a much larger scale than Africa: the nominal scale of such a map can be true on the equator, or on two parallel circles placed symmetrically around the equator. See also local scale and scale factor.
scale factor, local
For each point on a map, and a specified direction through that point, the local scale factor is the ratio between the local scale and the nominal scale. (If the map projection is conformal, the direction does not matter.) For example, let a MercatorProjection map have nominal scale 1 : 100 000, which is true on the equator. Then, for each point on latitude 60°, the local scale is twice as large, 1 : 50 000 (independent of direction). Then, the local scale factor is 2.0 on latitude 60°. If you zoom in this map, the scales change, but the local scale factor at each point remains constant.
In Carmenta Engine, you can compute two of the local scale factors at a point p, on a map drawn with the Crs R, like this: First make sure that p is expressed in projected coordinates. Then, let (xN, yN) be the result of calling R.NominalMetersXY(p), and let (xM, yM) be the result of calling R.MetersXY(p). The local scale factor at p, rightwards on the map, is now xN / xM. And the local scale factor at p, upwards on the map, is yN / yM. However, these two are not necessarily the maximal and minimal of all local scale factors at p.
scale factor, nominal
The property scaleFactor, which can be set on many kinds of Projection in Carmenta Engine, can be called the nominal scale factor of the projection. It equals the local scale factor at a central point of the projection. The central point is normally given by the central longitude and the central latitude but if the projection has a latitude of standard parallel, then the center point is any point on a standard parallel.
When defining a conformal projection, it is common to use a nominal scaleFactor that is slightly less than 1, since this will make the average of the local scale factors (in the entire map) nearer 1.
singleband raster
A raster where the cell values are one-dimensional, for example, numbers (a continuous raster), or values of a finite enumeration (a discrete raster), or an index into a color table (an indexed raster). See also multiband raster.
small circle
On a sphere, a small circle is a circle on the surface that is smaller than a great circle. On an ellipsoid, a small circle could mean all points that are at the same distance from the circle center, measured along the surface.
spatial index
Auxiliary data that makes it possible to quickly find a certain portion of a DataSet. The spatial index may consist of one or more auxiliary files, or it may be an intrinsic part of the data format.
spheroid
A spheroid is an ellipsoid of rotation. In geodetic literature, spheroid and ellipsoid can be considered synonyms. The Carmenta Engine documentation uses the term ellipsoid.
standard parallel
Some map projections have rotational symmetry around the earth axis, for example cylindrical projections in equatorial aspect, or conic or azimuthal projections in polar aspect. Such symmetric projections will have one or two standard parallels, which usually means parallel circles whose scale is true. In other words, the local scale on a standard parallel equals the nominal scale of the map, which means that the local scale factor is exactly 1. However, the term is sometimes generalized (or perhaps abused). For example, a Lambert conformal conic projection can be constructed with a single standard parallel and a nominal scale factor less than 1 - then the local scale factor on the standard parallel equals the nominal scale factor, instead of 1. In Carmenta Engine, some projections (AlbersEqualAreaConicProjection, MercatorProjection, MillerCylindricalProjection, LambertConformalConicProjection, PolarStereographicProjection, EquidistantCylindricalProjection, and EquidistantCylindricalAngularProjection) have one or two properties called standardParallel or similar, which represent standard parallels in the generalized (or abused) sense.
A conic projection with two standard parallels is often said to be constructed from a secant cone; that is, a cone that intersects the earth at the two standard parallels. This is a useful lie, but it is not correct geometrically. If you want to construct a Lambert conformal conic projection with two standard parallels, and you start by constructing such a secant cone, then its top angle will be wrong.
thematic raster
Synonym: discrete raster. A singleband raster that contains integer values that represent a finite enumeration, like terrain types: WATER, FOREST, CITY, ...
tile
A medium-sized chunk of a raster, perhaps 1024 * 1024 cells, that is easy to extract from a raster stored on disk. The tile size, and whether tiles exist, depends on the format for storing rasters.
Each file may correspond to one tile.
One file may consist of many tiles.
The format may be untiled: a single file may contain one very large raster, stored row by row. Such a format usually gives slower retrieval of data.
writable dataset lock
A lock provided by each instance of a writable dataset that must be used to synchronize access to the dataset and all features inside of it. The recommended way to take and release the lock is to use the Guard class.