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Wasmer Consulting - NMPlot - User's Guide - Introduction to the Noise Model Grid Format

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.

Data Point Specification

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

Geographic Annotations

These NMGF sections specify map data stored in a grid.

Audit Trail

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

Predefined Optional Attributes

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

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.

File Control

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

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