diff options
| -rw-r--r-- | docs/source/operations/operations_computation.rst | 67 |
1 files changed, 38 insertions, 29 deletions
diff --git a/docs/source/operations/operations_computation.rst b/docs/source/operations/operations_computation.rst index 33c635b2..930044e3 100644 --- a/docs/source/operations/operations_computation.rst +++ b/docs/source/operations/operations_computation.rst @@ -117,37 +117,46 @@ Filtering and sorting of coordinate operations The last step is to filter and sort results in order of relevance. -The filtering -takes into account a minimum accuracy that the user might have expressed, an -area of use on which the coordinate operation(s) must apply and if the absence -of grids needed by some operations must result in discarding the operations. +The filtering takes into account the following criteria to decide which operations +must be retained or discarded: -The sorting algorithm is a set of heuristics. It is implement by the `operator ()` +* a minimum accuracy that the user might have expressed, +* an area of use on which the coordinate operation(s) must apply +* if the absence of grids needed by an operation must result in discarding it. + +The sorting algorithm determines the order of relevance of the operations we got. +A comparison function compares pair of operations to determine which of the +two is the most releavant. This is implemented by the :cpp:func:`operator ()` method of the SortFunction structure. -The following criteria are used in the following order (first listed, first applied): - -* put at top operations that can be expressed as a PROJ instanciable operation - (the database might list operations whose method is not (yet) implemented by PROJ) -* put at top operations that do not have a synthetic ballpark vertical transformation - (occurs when there is a geoid model) -* put at top operations that do not have a synthetic ballpark horizontal tranformation -* put at top operations that refer to shift grids that are locally available -* put at top operations that refer to grids that are available in one of the proj-datumgrid - packages, but not necessarily locally available -* put at top operations that have a known accuracy -* if two operations have unknown accuracy, then put at top the one that use grid - if the other one does not (grid based operations are assumed to be more precise - than operations relying on a few parameters) -* put at top operations whose area of use is larger (note: the computation of the - are of use is approximate, based on a bounding box) -* put at top operations that have a better accuracy -* in case of same accuracy, put at top operations that do not use grids (operations - that use only parameters will be faster) -* put at top operations that involve less transformation steps -* and for completness, if two operations are comparable given all the above criteria, - put at first the one that has the shorter name, and if they have the same lengt, sort - by lexicographic order (obviously completely arbitrary, but a sorting - algorithm must be able to compare all entries) +When comparing two operations, the following criteria are used. The tests are +performed in the order they are listed below: + +1. consider as more relevant an operation that can be expressed as a PROJ operation string + (the database might list operations whose method is not (yet) implemented by PROJ) +2. if both operations evaluate identically with respect to the above criterion, + consider as more relevant an operation that does not include a synthetic + ballpark vertical transformation (occurs when there is a geoid model). +3. if both operations evaluate identically with respect to the above criterion, + consider as more relevant an operation that does not include a synthetic + ballpark horizontal tranformation. +4. consider as more relevant an operation that refers to shift grids that are locally available. +5. consider as more relevant an operation that refers to grids that are available + in one of the proj-datumgrid packages, but not necessarily locally available +6. consider as more relevant an operation that has a known accuracy. +7. if two operations have unknown accuracy, consider as more relevant an operation + that uses grid(s) if the other one does not (grid based operations are assumed + to be more precise than operations relying on a few parameters) +8. consider as more relevant an operation whose area of use is larger + (note: the computation of the are of use is approximate, based on a bounding box) +9. consider as more relevant an operation that has a better accuracy. +10. in case of same accuracy, consider as more relevant an operation that does + not use grids (operations that use only parameters will be faster) +11. consider as more relevant an operation that involves less transformation steps +12. and for completness, if two operations are comparable given all the above criteria, + consider as more relevant the one which has the shorter name, and if they + have the same length, consider as more relevant the one whose name comes first in + lexicographic order (obviously completely arbitrary, but a sorting + algorithm must be able to compare all entries) Geodetic/geographic CRS to Geodetic/geographic CRS, without known identifiers ----------------------------------------------------------------------------- |