Anchor: #CCHBHBHE

Section 7: Metadata

Anchor: #i1065816

Identifying Delivered Coordinates

All coordinates files or lists delivered, whether hard copy or in digital medium, must contain metadata indicating the CAF (or SAF), horizontal datum and adjustment, vertical datum (geoid model if applicable), units of measure and the date of the field work. This would include hard copy drawings, CAD drawings, the data sheets, and each sheet containing coordinates in a report, and ASCII or LandXML files.

Anchor: #i1065850

Conversions and Transformations

Where design survey accuracy is required, TxDOT will not accept any datum transformations. There is no way to accurately transfer NAD27 coordinates to NAD83 datum. CORPSCON and other conversion software programs are based on NADCON algorithms, which perform a rubber sheeting adjustment that is not accurate.

If a change to the NAD83 project datum is needed for comparison of old surveys, two (2) control points can be resurveyed (GPS or conventional) from references in the new datum. Then, a translation-rotation-scale can be done holding to the two (2) points common to both datum. Or, if original raw GPS data is available, it can be reprocessed holding the new datum coordinates rather than the original datum coordinates.

Both CORPSCON and AASHTOWare SDMS Processor are acceptable software programs for the mathematical conversion of:

  1. Metric to US Survey Foot (or visa-versa)
  2. SPC zones to adjoining SPC zones
  3. UTM to SPC’s and
  4. Latitude/Longitude to SPC’s.

In addition to these strictly mathematical conversions, these software programs provide useable combined adjustment factors (CAF) at the specific location of a point if the elevation is included in the input.

Previous page  Next page   Title page