9.10.7 How do I prevent corruption from conversion errors?

In many cases, datum transformations can have errors of several meters or more. Be cautious about using any transformation in an absolute sense. Transformations should only be used for relative corrections. Care should also be taken that the same correction be used throughout. Here are several tips on minimizing these effects:

•Process in the local datum (for example, NAD83, ED50, and so on) as much as possible. This methodology effectively uses a relative datum correction and minimizes the possibility of using multiple datum conversions. Processing in the local datum is performed by selecting the processing datum under

Settings | Individual | General. Click on the More Datum Options… button to ensure that the Use processing datum by setting processing datum to local datum option is enabled.

•For localized datums that is, datums with large rotations, scale or shifts, or for NADCON corrections, processing should use a more global datum (for example, WGS84 or NAD83). It is very important that the same conversion be used in the coordinate input and for the Export Wizard. This entails checking the conversion being used. In addition, avoid using the Write Coordinates feature to export coordinates, as it does not support input datums different from the processing datum.

•If using a newer version of GrafNav or GrafNet, or if examining an old project, be sure that the default conversion has not changed.

•Ιf using an external program to transform coordinates to the processing datum (for example, WGS84), then use the same program to convert the final coordinates back to the local datum.

•Be cautious with geoids. Geoids designed for one datum may not be compatible with another.

9.11Projections FAQ and Tips

A map projection is the representation of the earth’s spherical surface on a flat surface. Map projections are also sometimes referred to as grids. UTM is one of the most common map projections. Waypoint’s software currently support the following map projections:

Transverse Mercator

This employs a cylinder placed horizontally around the earth. Transverse Mercator (TM) has little distortion in the north-south direction, but the scale factor deviation from unity grows along with the convergence with distance from the central meridian. UTM starts a new zone every 6 degrees to circumvent this.

Lambert Conformal

This utilizes a cone placed over the north (or south) poles. Distortion is minimal in the east-west direction, while the limitations are in the north-south direction.

Oblique Mercator

This is similar to the TM in the sense that it employs a cylindrical projection. The difference is that the cylinder is oblique, running along the great circle arc between any two points. The result is a projection with little distortion in the direction of the chosen arc.

Local Cartesian

See Section 9.8.5, on Page 280 for information.

Local Coordinate Grid

See Section 9.8.6, on Page 281 for information.

US State Plane

New Zealand Mapping Grid (NZMG)

Earth-Centred Earth-Fixed (ECEF) Stereographic

Projection of the Earth onto a flat plane. It is similar to the Local Cartesian project in many ways, but it is better suited to larger areas and scaling is handled differently.