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Section 6: Units and Datum

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Units

Unless otherwise instructed, latitude and longitude will be presented as degrees, minutes, and seconds. Direction indicators N or W will prefix the value and seconds will be carried out five places right of the decimal where accuracy is to approximately .001 feet.

The coordinate system used by TxDOT is the State Plane Coordinate System in NAD83; however, units of length will be in U.S. Survey Feet rather than meters. Horizontal coordinates should be carried out to .001 feet, unless otherwise instructed.

Processing and adjusting GPS data may be done in the metric system but all project data must be delivered in U.S. Survey Feet.

Conversion from meters to U. S. Survey Feet must be made using the following formula:

Meters * 3937/1200 = U. S. Survey Feet

The factor is 3.280833333333 and working with SPC’s in the millions, one must carry the factor out to 12 places to the right of the decimal as shown.

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Datum

All geodetic surveying with GPS will be done in the NAD83 horizontal datum. An adjustment was done in Texas (using GPS), which resulted in the 1993 HARN network. The network was extended to nearly all old, conventionally surveyed federal monumentation. Projects should be referenced to the published HARN coordinates of NGS monumentation.

Elevations will be referenced to the NAVD88 vertical datum.

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Surface Coordinates vs. State Plane Grid Coordinates

Depending on how far north or south the project falls in the state plane zone and depending on the elevation of the area; GPS coordinates in the State Plane Coordinate System most likely will need to be adjusted so that lengths measured on the surface will coincide with lengths inversed on the surface projection (state plane grid). An exception to this is when RTK work is done after calibrating the equipment to control in already existing surface coordinates.

The TxDOT surveyor or engineer for the project may calculate a combined adjustment factor (CAF) to be used on the project. The surveyor or engineer also might dictate that a standard, county-wide TxDOT Surface Adjustment Factor (SAF) be used or may ask a consultant to calculate their own. It is not important what method is used to arrive at the factor, but it is absolutely necessary that the factor used is included in the metadata notes.

The factor should be carried out to eight (8) places in order that all involved parties arrive at the same coordinates to the nearest thousandth of a foot. Seven (7) places will yield only hundredths; the common nine (9) places used by NGS will produce adjusted coordinates to the ten thousandths of a foot.

Highway projects with several CAF’s pose no problem when all coordinates can be backed down to SPC’s so everything will match. The juncture of two systems; however, needs to be well identified so that all measurements stop or begin at a common point. Stationing must not cross this line without a station equation. Cut and fill volumes can’t be calculated across the line. Only State Plane Coordinates can be used seamlessly.

If coordinates have been truncated for easier calculations or for identification, they must be returned to their full configuration before delivery. This is not only for standardization; it is so that the coordinates will work in the seed files for MicroStation as well.

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