Geo-Modeling or 3D Model or Height Model

Geo-Modeling or 3D Model or Height Model

Atriyon Julzarika

I. Digital Surface Model (DSM)

1. A DSM is an elevation model that includes the tops of buildings, trees, powerlines, and any other objects. Commonly this is seen as a canopy model and only 'sees' ground where there is nothing else overtop of it.
2. Essentially the full point cloud, with ground, structures, and vegetation. (cheapest)
3. Digital surface model (DSM ) – a first-reflective-surface model that contains elevations of natural terrain features in addition to vegetation and cultural features such as buildings and roads.

II. Digital Elevation Model (DEM)

1. A DEM is a 'bare earth' elevation model, unmodified from its original data source (such as lidar, ifsar, or an autocorrelated photogrammetric surface) which is supposedly free of vegetation, buildings, and other 'non ground' objects.
2. Incidentally, a DEM is far cheaper to produce an a DTM.
3. Bare Earth with structures and vegetation removed
4.  Digital Elevation Model (DEM ) – a bare-earth model that contains elevations of natural terrain features such as barren ridge tops and river valleys. Elevations of vegetation and cultural features, such as buildings and roads, are digitally removed.
5. Elevation: “height above a given level, especially that of sea”; “height above the horizon”; etc.
6. the terms elevation emphasize the “measurement from a datum to the top” of an object. elevation do not necessarily refer to the altitude of the terrain surface, but in practice, this is the aspect that is emphasized in the use of these terms.
7. DEM was widely used in America

III. Digital Terrain Model (DTM)

1. A DTM is effectively a DEM that has been augmented by elements such as breaklines and observations other than the original data to correct for artifacts produced by using only the original data. This is often done by using photogrammetrically derived linework introduced into a DEM surface. An example is hydro-flattening commonly seen in elevation models done to FEMA specifications
2. DEM with thinned ground points (mass points) and breaklines. (most expensive)
3. the DTM was defined as a digital (numerical) representation of the terrain.
4. Terrain: “tract of country considered with regarded to its natural features, etc.”; “an extent of ground, region, territory”; etc.
5. The meaning of “terrain” is more complex and embracing. It may contain the concept of “height” (or “elevation”), but also attempts to include other geographical elements and natural features. Therefore, the term DTM tends to have a wider meaning than DHM or DEM and will attempt to incorporate specific terrain features such as rivers, ridge lines, breaklines, etc. into the model.
6. Generally, a DTM could contain the following four groups of (topographic and non topographic) information as follows:

- Landforms, such as elevation, slope, slope form, and the other more complicated geomorphological features that are used to depict the relief of the terrain.

- Terrain features, such as hydrographic features (i.e., rivers, lakes, coast lines), transportation networks (i.e., roads, railways, paths), settlements, boundaries, etc.

-     Natural resources and environments, such as soil, vegetation, geology, climate, etc.

-  Socioeconomic data, such as the population distribution in an area, industry and agriculture and capital income, etc.

7. A DTM is an ordered set of sampled data points that represent the spatial distribution of various types of information on the terrain. The mathematical expression could be something like: KP = f (uP , vP ), K = 1, 2, 3, . . . ,m, P = 1, 2, 3, . . . , n (1.1) where KP is one attribute value of the kth type of terrain feature at the location of point P (which can be a single point, but is usually a small area centered by P); uP , vP is the 2-D coordinate pair of point P; m (m ≥ 1) is the total number of terrain information types; and n is the total number of sampled points. For example, suppose soil type is categorized as ith type of terrain information, then the DTM of this component is expressed as IP = fi(uP , vP ), P = 1, 2, 3, . . . , n. (1.2)
8. A DTM is a digital representation of the spatial distribution of one or more types of terrain information and is represented by 2-D locations plus a mathematical representation of terrain information. It is commonly regarded as a 2.5-D representation of the terrain information in 3-D geographical space.

IV. Digital Ground Model (DGM)

1. Ground: “the solid surface of the earth”; “a solid base or foundation”; “a surface of the earth”; “bottom of the sea”; etc.
2. A DGM more or less has the meaning of “a digital model of a solid surface.”
3. DGM was used in the United Kingdom.

V. Digital Height Model (DHM)

1. Height: “measurement from base to top”; “elevation above the ground or recognized level, especially that of the sea”; “distance upwards”; etc.
2. the terms height emphasize the “measurement from a datum to the top” of an object. Height do not necessarily refer to the altitude of the terrain surface, but in practice, this is the aspect that is emphasized in the use of these terms.
3. DHM came from Germany

VI. Digital Terrain Elevation Model (DTED)

1. DTEM was introduced and used by USGS and DMA (Defense Mapping Agency)
2. Terrain: “tract of country considered with regarded to its natural features, etc.”; “an extent of ground, region, territory”; etc.
3. Elevation: “height above a given level, especially that of sea”; “height above the horizon”; etc.

VII. Geoid

1. The term Geoid is used to portray the shape of the Earth's surface, and it identifies that surface to which the oceans would conform over the entire Earth if free to adjust to the combined effect of the Earth's mass attraction and the centrifugal force of the Earth's rotation.
2. Among the Earth’s equipotential surfaces, the Geoid coincides with the mean sea level of the oceans through a process of Least Squares Approximation. The Geoid extends under the continents and differs from an ellipsoid of revolution by vertical distances that are within the order of one hundred metres.
3. Ignoring for the moment that soundings on charts are referred to a low water chart datum, the Geoid is the reference surface for heights (orthometric or dynamic, defined below) used in mapping. As such it is often called a "vertical datum" and the heights referred to it are commonly known as "heights above mean sea level". The practical realization of the vertical datum is normally achieved by accepting a mean sea level at the locations of tide gauges along the seashore. This realization carries with it some inherent errors that may reach well over one metre. The local mean sea level is determined indirectly, by studying the tide-gauge record for a certain time period and is thus tacitly valid for that time period.