{"id":11033,"date":"2024-01-12T01:50:20","date_gmt":"2024-01-12T00:50:20","guid":{"rendered":"https:\/\/www.aboutriver.com\/?p=11033"},"modified":"2024-01-12T01:50:20","modified_gmt":"2024-01-12T00:50:20","slug":"how-to-convert-navd88-to-ngvd29-on-the-mississippi-river","status":"publish","type":"post","link":"https:\/\/www.aboutriver.com\/how-to-convert-navd88-to-ngvd29-on-the-mississippi-river\/","title":{"rendered":"How To Convert Navd88 To Ngvd29 On The Mississippi River"},"content":{"rendered":"
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Identifying the Datums<\/h2>\n

When attempting to convert coordinate systems, the first step is to identify the datums used in each. Datums are fundamental to any coordinate system and a reliable understanding of each is required. The Mississippi River utilizes two datums, NAD83 (North American Datum 1983) and NGVD29 (National Geodetic Vertical Datum 1929). They both use the State Plane Coordinate System (SPCS), which is based on a plane-rectangular-coordinate system. In the Mississippi River, NAD83 is the basis for the horizontal datum while NGVD29 is the basis for the vertical datum.<\/p>\n

Differences between NAD83 and NGVD29<\/h2>\n

NAD83 is more accurately based on the in-plane-fit to the North American continent and its physical features. On the other hand, NGVD29 is based on the Great Lakes-St Lawrence and Atlantic coasts. While these features have been measured quite accurately, both datums tend to drift from the true datum, resulting in the need for conversion.<\/p>\n

Coordinate System Conversion Process<\/h2>\n

The process for converting coordinates from one datum to another is established by the North American Datum Conversion Factors Working Group (NADCFWG). The NADCFWG is a group of experts in the field of surveying and mapping that provides a database and guidance on how to convert coordinates. The conversion process requires two sets of data points and then the NADCFWG database is used to establish a two-dimensional transformation matrix. The two data points are then used to convert the points from NAD83 to NGVD29.<\/p>\n

Transformation between NAD83 and NGVD29<\/h2>\n

The transformation between NAD83 and NGVD29 along the Mississippi River tends to drift northward. This is due to the fact that the NAD83 datum is moving westward at a faster rate than the NGVD29 datum. This westward drift creates a difference between the two, especially when considering the large distances of the Mississippi River.<\/p>\n

Using the NADCFWG Database<\/h2>\n