About Geoid Models

A geoid model represents a specific mathematical representation of the geoid. It determines the geoid's height above a reference ellipsoid at any given location. These models are constructed using diverse data sources, including satellite measurements, gravity observations, and oceanographic data. They offer a practical method for approximating the geoid surface, facilitating applications in fields such as surveying, mapping, and navigation.

Supported geoid model types

• BIN. National Geodetic Survey (NGS) .bin file
• DAT. EGM Geoid Models
• ISG/ISG2. International Service for the Geoid
• OSGM. Ordnance Survey Geoid Model 2015
• GRI. OSGM91
• BYN. NRCan .byn format

Geoid models included by default

• EGM96 Earth Gravitational Model 1996 Specification
• GEOID12B Geoid Model for North America (2012) Specification
• GEOID18 Latest Geoid Model for North America (2018) Specification
• GEOID03 Geoid Model for North America (2003) Specification

How to Add Geoid Models

Adding Geoid Models for use in the Vertical Datum Transformation Tool is a manual process working in the Geoid Models directory: C:\ProgramData\Autodesk\C3D <year>\enu\Geoid Models.

Manual geoid model configuration

1. In the Geoid Models directory, locate the Geoid Models.json file.
2. Create a back up of the file.
3. Open the original Geoid Models.json file in a text editor.
4. Using the file schema (see the About the Geoid Models.json Schema section below), add a new geoid model entry.
5. Save the file.
6. The geoid model is now available in the Vertical Datum Transformation Tool.

Automatic geoid model configuration

This method is dependent on the geoid model type. Refer to the Supported Grid Model Chart below for more details.

1. In the Geoid Models directory, locate the Geoid Models.json file.
2. Create a back up of the file.
3. Copy the new grid file (or folder) to the Geoid Models directory.
4. Delete the Geoid Models.json file.
5. Run the Vertical Datum Transformation Tool to regenerate an updated file including the new grid file information.
6. The geoid model is now available in the Vertical Datum Transformation Tool.

Supported Grid Model Chart

About the Geoid Models.json Schema

The Geoid Models.json file defines the geoid models loaded by the Vertical Datum Transformation Tool. This file contains information about the available geoid models, their display names, file paths, and associated metadata. The following schema example provides field definitions.

"g2003a01.bin": {
    "DisplayName": "g2003a01.bin",
    "Name": "g2003a01.bin",
    "FileName": "<absolute path to g2003a01.bin>",
    "Datum": "NAD83",
    "MinPoint": [
      	60,
      	172
    ],
    "MaxPoint": [
      72.01666666666667,
      -155.98333333333335
    ],
    "GeoidModelType": 4,
    "DeltaLat": 0.016666666666666666,
    "DeltaLon": 0.016666666666666666,
    "North": 259260,
    "South": 216000,
    "East": 734460,
    "West": 619200,
    "DeltaLatI": 60,
    "DeltaLonI": 60,
    "NumberLat": 721,
    "NumberLong": 1921,
    "ByteCount": 0,
    "LittleEndian": false,
    "NoData": 0
  	},

Field descriptions

• DisplayName. The geoid model name displayed in the Vertical Datum Transformation.
• Name. The internal name of the geoid model.
• FileName. The absolute path to the geoid model file.
• Datum. The datum associated with the geoid model (such as NAD27, NAD83, or WGS84).
• MinPoint. The minimum geographic latitude and longitude coordinates covered by the geoid model.
• MaxPoint. The maximum geographic latitude and longitude coordinates covered by the geoid model.
• GeoidModelType. The numeric type of geoid model.
  1. DAT
  2. ISG
  3. ISG2
  4. BIN
  5. OSGM
  6. Gri
  7. Byn
• DeltaLat. The latitude increment between grid points in the geoid model.
• DeltaLon. The longitude increment between grid points in the geoid model.
• North. The northernmost extent of the geoid model in grid units.
• South. The southernmost extent of the geoid model in grid units.
• East. The easternmost extent of the geoid model in grid units.
• West. The westernmost extent of the geoid model in grid units.
• DeltaLatI. The latitude increment in grid units.
• DeltaLonI. The longitude increment in grid units.
• NumberLat. The total number of latitude grid points in the geoid model.
• NumberLong. The total number of longitude grid points in the geoid model.
• ByteCount. The number of bytes used by the geoid model file.
• LittleEndian. A boolean value indicating whether the file uses little-endian byte order.
• NoData. The value used to represent missing or no data in the geoid model.