Unit

data items and units

The GeoDMS calculates with data items. A data item represents a mapping between a domain-unit set and a value set. These sets are represented in the GeoDMS by unit items. For configured data items, the domain unit (attribute only) and values-unit need to be configured explicitly. The units are used to check consistencies in calculations.

Unit consistency checks are an important feature of the GeoDMS to improve the quality of models, as modelling errors are often related with incorrect use of units and related conversion factors. See also http://en.wikipedia.org/wiki/Units_conversion_by_factor-label.

For each operator and function it is specified if and how the units of the arguments must relate. The add operator can e.g. not meaningfully combine values expressed in meters with values expressed in seconds. These values can however be meaningfully combined with the divide operator, resulting in a velocity attribute with values expressed in meter per second. More information on this topic can be found on the page dimensional analysis.

syntax

Each unit is configured with the key word: unit. The element type is indicated between the less than (<) and greater than (>) characters, followed by the name of the unit e.g.

unit<uint32> municipality2022: nrofrows = 335

This results in a unit with 335 entries, to be used for all municipalities in 2022 in the Netherlands.

roles

A GeoDMS unit can have two roles:

  1. domain-unit
  2. values-unit

This is similar to links forming a network by having an origin node and a destination node. The domain unit specifies the source of the mapping (entity) represented by a data-item. The domain unit must be countable with a defined range (i.e. integer or a 2D rectangular integer rasterpointset or void). The values unit indicates in which value type and metric the values of a data items are expressed. 2D values (points, lines, polygons) can be a Projection, i.e. two affine translation of a basic coordinate System.

It is important to understand that the same unit can have multiple roles in one configuration. Assume we have a set of houses and a set of regions. The domain_unit house and region are configured, indicating the number of elements and order for the attributes of these domains. If we make a relation which defines for each house in which region it is located, this would result in an attribute with as domain unit house and as values unit: region. So the unit region is used for both roles.

value types

Values can be:

  • one-dimensional
  • two-dimensional
  • a single element
  • sequence of elements

The elements can be (un)signed, integer or float, or utf8-text-string (implemented as a sequence of characters).

Each value has an element type that determines how its element(s) is/are represented in memory and which determines its minimum and maximum value and its granularity. operators-and-functions are implemented to operate on data items of (combinations of) value types.

Usually, different combinations are dealt with by different instantiations of an operator/function, to avoid time on processing value type conversion or larger than required memory-footprints. With conversion-functions, data items can be converted to other values units and value types.