By using this window you can create a transformation model between two coordinate reference systems (source and destination). A transformation model is composed of one or more transformation steps that are generated according to the operations with coordinates of a route. The route is formed from a succession of CRSs that are connected between them. The connection between two CRSs represents an operation with coordinates through which the passing from a coordinate reference system to another is made. The description of a route is presented in the figure below:

• CRS_S: CRS source;
• CRS₁: Intermediate CRS with the ordinal number 1;
• CRS_n: Intermediate CRS with the ordinal number n;
• CRS_D: CRS destination;
• O_i: operation with coordinates with the ordinal number i, i=1..n+1.

Fig. Scheme of a route identified from the EPSG database

Reading information from the EPSG database

The possible routes between two CRSs are identified using operations with coordinates defined in the table Coordinate_Operation from the EPSG database. From this table the information about the source CRS and the target CRS is used in order to made the connection between two CRSs. For the operations that don’t have defined the source and target CRS their identification is made using the table Coordinate Reference System of the database from which the CRS that uses the operation is identified and the base CRS thereof. Therefore, the operation will have as source CRS the base CRS and the target CRS the CRS that uses the operation.

The operations that have many variants are integrally taken.

Note: If between two given CRSs a direct operation is found (without intermediate steps) it should be used before any other indirect route (with intermediate steps).

Note: The transformation steps are created according to the EPSG indications referring to the use of operations with coordinates and to the methods used by them.

Recommendation: Verify the transformation steps generated by this command. The parameters taken from the database for projections, 2D transformations, 3D transformations or polynomial transformations can be incorrect because it is possible that in the current version of the database these might not be up to date.

Start / Destination Coordinate Reference System (CRS)

In this box summary information is presented about the selected start / destination CRS.

CRS Code

Type the code or press the Select CRS ... button.

Button Select CRS ...

Choose from the database the start / destination CRS.

Information about the selection mode of the CRS can be found at The window for selecting the Coordinate Reference System (CRS) from the EPSG database .

Button Invert systems ...

It reverses the two CRSs, the start and destination one, in order to create the inverse transformation model.

Filters

By using these filters you can reduce the number of possible routes between the two CRSs.

Accept operations only in area of start and destination CRS

It sets if when searching for routes are loaded only the operations with coordinates whose area of use intersects with the area of use of the two CRSs. If this filter is not used, all the operations with coordinates are taken over regardless of the area of use.

Maximum accuracy for a coordinate operation

Introduce in meters the value of the maximum accuracy. If this filter is used, only the coordinates operations that have the accuracy smaller or equal to this value will be taken from the database.

Note: This value does not represent the total accuracy of a route, but the maximum accuracy of a coordinate operation from two CRSs.

Maximum number of operations between start and destination

Introduce the maximum number of coordinates operations (steps). If this filter is used, only the routes that do not exceed this number of steps will be searched for. If it is not used only the routes of maximum 8 steps will be searched for.

Buttons

Button Search routes

When calling this button, a search of possible routes between the start and target CRS is made. The search is made according to the settings from the filter box.

The identified routes are loaded in the list of routes. When nodes are loaded they are sorted in ascending ordered according to the number of nodes attached. If two or more nodes have the same number of nodes attached the ordering is ascending according to the accuracy of the operation between the parent node and the one that needs to be sorted. If the accuracy is the same, the sorting is done in descending order according to the operation's revision date. If and the revision date is the same, the sorting is done in descending order according to the operation's variant number.

Once the nodes from the list have been loaded, the program selects the shortest route (the route with the fewer steps). If there are more routes with the same number of steps, the route with the best total accuracy is selected.

The total accuracy is calculated using the formula A_T = (A₁²+ A₂² + ... + A_n²)^1/2, where:

• A_T – total accuracy;
• A₁, A₂, ... A_n – the accuracy of the operations with coordinates of a route.

Note: It is possible that the automatically selected route does not correspond with the route you want; in this case manually select the route that you want. If in the list of routes you can’t find a route similar to the one you want, the causes that can make this route not to exist are: the used filters eliminated the respective route or in the database there isn’t defined an operation with coordinates between CRSs that you want the route to pass through.

Button Stop ...

With this button you can stop the search started by calling the button Search routes.

Button Clear

Delete all the nodes from the list of routes.

Routes

The list of routes found between the start and destination CRSs. The list is tree type and it is composed of nodes connected between them. Each node from this list represents a CRS and the link between two nodes represents an operation with coordinates.

When selecting a node, in the list of Operation description: ... will be shown information about the operation with coordinates between the parental node of the selected node and the selected node.

A selected route is valid for creating a transformation model only when a final node is selected (destination CRS) from this list. In this case the nodes that form the route are marked with green.

Selected Route

When a final node is selected (destination CRS) from the list of routes, the route and the total accuracy of the selected route are presented in this box.

Operation description: ...

Shows detailed information referring to the selected operation. The operation is the one between the parental node of the selected node and the selected node.