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SQL Server Tips by Gama and Naughter Consulting



Similar to the XP_ELLIPSOIDINFO XP, the XP_GIS DLL also requires a database of datums for some of its functionality. Each datum is defined by the following structure:

struct CReferenceDatum
char* Name;
ReferenceEllipsoid elipsoid;
double dx;
double dy;
double dz;
double ex;
double ey;
double ez;
long Satellites;

The “Name” is a string, which identifies the datum. The ellipsoid value is the reference ellipsoid on which this datum is based. The “dx”, “dy” and “dz” values define translation values required when converting to the GPS datum, which is called WGS 1984. These values have been calculated by obtaining the latitude and longitude of various datum points referred to the WGS datum. The latitude and longitude points of the datum point in the original datum system will of course be already known. From these coordinates, the dx, dy and dz values can be determined. Sometimes these values can be in the order of hundreds of meters. This explains how the coordinates of a point can change by this amount when a map is redrawn using a new datum. The “ex”, “ey” and “ey” values give an indication of the errors involved in the transform. The “Satellites” value is the number of measurement stations used to obtain the transformation values. A database of 219 datums is stored in the DLL. The list of datums is based on the list provided on Peter H. Dana at The various ID’s are defined by an enum called “ReferenceDatum”.

This XP takes 10 parameters. The first parameter is the ID of the datum to return results for. The other 9 parameters are output parameters and correspond to the values in the “CReferenceDatum” structure. The XP output parameters are in the same order as they are defined in the structure.

The above book excerpt is from:

Super SQL Server Systems
Turbocharge Database Performance with C++ External Procedures

ISBN: 0-9761573-2-2
Joseph Gama, P. J. Naughter





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