NMPlot User's Guide, Appendix F: Introduction to the Noise Model Grid Format

The Noise Model Grid Format (NMGF) is a well-documented standard way of representing a grid and its auxiliary information.

Internally, NMPlot processes all grids using NMGF, as shown in the Details pane of the Grid Document window.

The NMGF standard includes a format for storing grids in files. Many third-party applications communicate with NMPlot using NMGF grid files. Writing a NMGF file is the easiest ways to import third-party data into NMPlot. For a quick-start guide to creating NMGF files for this purpose, see Quick-Start Guide to Importing Data into NMPlot.

Full Documentation for NMGF

This appendix provides an overview of the NMGF standard. Complete documentation is available from the NMGF web page, http://www.wasmerconsulting.com/nmgf.htm.

Why Was NMGF Developed?

There are a number of computer models in use that calculate the noise impacts of installations, such as airports and railroads, upon the surrounding community. Examples of such models include the United States Air Force's (USAF) NOISEMAP and the United States Federal Aviation Administration's (US FAA) Integrated Noise Model (INM).

In the early 1990's, each model defined its own output file format. This resulted in duplicate effort developing post-processing utilities that performed essentially the same tasks. For example, both the USAF and the US FAA independently developed contour plotting utilities for use with their respective noise models.

To promote the development of shared post-processing utilities, the USAF and the US FAA sponsored the development of a standard grid file format that met the needs of both models. This format is known as the Noise Model Grid Format (NMGF). Version 1.0 of this format was released in September 1993.

NMGF: Not Just For Environmental Noise Models

While NMGF was originally developed to support noise models, it is neutral to any particular application domain. A NMGF grid can just as easily contain data from other types of models (for example, air pollution models) or from sources other than computer models (for example, measured data).

NMGF Sections

Data is stored in the NMGF format as a list of sections. There are more than 30 types of sections, each with a specific purpose. For example, a CART section defines a coordinate system used to specify geographic locations in a grid.

Section types are defined that specify data points (both on rectangular grids and as lists of location-value pairs), information about data points (the physical units, the range of valid data point values, etc.), geographic annotations (for example, the locations of roads and buildings), and bookkeeping information (where the data came from, who created the data, etc.).

Child Sections

The NMGF data structure is hierarchical. That is, sections can have subsections, called child sections. A child section specifies an attributes of its parent section.

As an example, consider the ZCRD section, which specifies an altitude or height. A ZCRD section by itself is meaningless: you do not know what the altitude refers to. The ZCRD section is always used as a child of the section it refers to.

Consider the PNTS section, which specifies information about a point geographic feature (for example, the location of a house). This section has several required parameters, such as the geographic coordinates of the point. The altitude of the point is not a required parameter. However, the ZCRD section can optionally appear as a child of the PNTS section to specify the altitude of the point.

Types of NMGF Sections

Geographic Coordinate System Specification

This NMGF section defines a geographic coordinate system used to specify locations in the grid. If a grid does not contain a CART or UTMC section, longitude and latitude are assumed.

• CART - Define a local flat-Earth Cartesian coordinate system for specifying geographic coordinates

• UTMC - Define a Universal Transverse Mecator (UTM) coordinate system for specifying geographic coordinates

Data Point Specification

These NMGF sections specify information about the data points in a grid.

• DAPY - Specify the grid's defined area polygon: i.e., the geographic area where data is known. See Defined Area Polygon.

• DPAL - Specify a group of data points as a list of location-value pairs.

• GRID - Specify a group of data point arranged on a regular rectangular grid

• GTSH - Specify the range of data point values that is considered valid

• MTRC - Specify what is being measured by the data points, and what units of measurement are used

• SUBG - Specify a nested subgrid: i.e., a set of data points that doubles the resolution of a portion of a grid

Geographic Annotations

These NMGF sections specify map data stored in a grid.

• AREM - Define a geographic feature consisting of one or more closed polygons

• ARES - Define a geographic feature consisting of a single closed polygon

• BKMP - Define information about a background map that is stored in an external file

• LINC - Define a geographic feature consisting of a series of (possibly non-contiguous) lines and arcs, described by a series of commands that move a hypothetical cursor

• LINM - Define a geographic feature consisting of one or more continuous lines

• LINS - Define a geographic feature consisting of a single continuous line

• PNTM - Define a geographic feature consisting of one or more points

• PNTS - Define a geographic feature consisting of a single point

Audit Trail

This NMGF section is used to record the creation and modification history of a grid.

• SORC - Specify audit trail information about a grid: how, why, when, where, and by whom it was created

Predefined Optional Attributes

These NMGF sections allow predefined optional attributes to be specified. They are used only as child sections.

• DATE - Specify a date

• DESL - Specify a detailed multiple-line description of an object

• DESS - Specify the proper name or a short one-line description of an object

• HEAD - Specify the direction (i.e., the heading) of an object

• PERS - Specify information about a person: name, telephone number, etc.

• PROG - Specify the name and version of a computer program

• TIME - Specify a time

• WARN - Specify a problem or limitation of the data in a NMGF grid

• ZCRD - Specify the height or altitude of an object

Arbitrary Optional Attributes

These NMGF sections allow grids to store information that was not anticipated by the designers of the NMGF format: arbitrary attributes can be specified as name-value pairs. These sections are used only as child sections.

• ATRC - Specify a coordinate as an arbitrary attribute of a NMGF section

• ATRF - Specify a floating point number as an arbitrary attribute of a NMGF section

• ATRI - Specify an integer as an arbitrary attribute of a NMGF section

• ATRS - Specify text as an arbitrary attribute of a NMGF section

• ATRT - Specify a table of information as an attribute of a NMGF section

File Control

These NMGF sections control the processing of a NMGF grid when it is stored in a file.

• ENDF - Mark the end of a NMGF grid file

• TITL - Mark the beginning of a NMGF grid file, and specify the NMGF version number

Simple Rectangular Grid Optimization

If a grid meets the following conditions, NMPlot can use alternative interpolation and contouring algorithms that are faster and use less memory.

• The grid contains a single GRID section, and does not contain any DPAL or SUBG sections.

• The grid either a) does not contain a GTSH section, or b) does not contain any invalid data points (i.e., data points whose values are outside the valid range specified by the GTSH section).

For large grids, the time and memory savings can be substantial. Therefore, if your data points are arranged on a regular, rectangular lattice of locations, they should be specified using a GRID section instead of a DPAL section.