This window allows including, editing or removing custom Earth Model in SPRING. The table shows, initially, both user created and default (hereafter system) datums. User created datums will be show at the beginning of the table, with system datums following. System datums are exhibited only for visualization. Through the button "Show Datuns" it's possible to toggle the visualization between User Datuns, System Datuns and both (default value).
Table fields are: (1)Projection; (2)Name; (3)Major semi-axies or equatorial radius; (4)Flattening; (5)X translation; (6)Y translation; and (7)Z translation. The Projection field is fixed and may have the following values: NOPROJECTION; UTM; MERCATOR; GAUSS; LAMBERT MILLION; LAMBERT; POLYCONIC; CYLINDRICAL; LATLONG; POLAR STEREOGRAPHIC; BIPOLAR OBLIQUE; ALBERS; MILLER; and SINUSOIDAL. The Name field may have any value. 3 to 7 parameters are calculated and published by Cartography and Geodesy offical agencies for each country. To add, edit or remove an Earth Model one must first enter Table Editing Mode through the button "Edit Table".
Add an Earth Model
- To add a new Datum, click first on "Add Datum" button. A new datum will be created as the first table item. After adding a datum, select its projection. When a projection is selected, major semi axis and flatting fields assume default values that may be changed later. One must give values to the other fields.
Edit an Earth Model
- Double click the item to be modified.
Remove an Earth Model
- To remove one or mroe Datuns select the corresponding table lines and click on "Remove Datum" button. To each datum to be removed SPRING will ask for confirmation. Notice it's not possible to remove system datums.
After editing modifications must be saved or will be lost.
It's important to notice that exists three situations where the translations field
may hold a null value.
(a) When a country official datum is used as reference to any other datum. In Brazil, for example, the current datum is SAD-69 and it is used as reference datum in SPRING. Because of this, it's parameters are 6378160, 3.352892e-03, 0, 0, 0. Any other datum must have translations from SAD-69 origens.
(b) When the reference datum is global, geocentric like WGS-84. In this case it's paramteres are 6378137, 3.352811e-03, 0, 0, 0. Any other datum must have translations from WGS-84.
(c) When a single ellipses is used as reference. For Hayford Ellipses, also know as 1924 International Ellipses, parameters must be 6378388, 3.367003e-03, 0, 0, 0.
Suppose a bolivian SPRING user wants to customise his "datum" file, in relation to UTM. She may use WGS-84 as
an reference and define the translation of the local datum, PSAD-56, in reference to WGS-84.
Remembering UTM has identifier 1, the modified file may have the lines below. In this example, the bolivian
user that chooses UTm project has disponible Earth Models WGS-84 and PSAD-56, besides Hayford and Clarke ellipses.
1
WGS84
6.378137e+06
3.352811e-03
0.000000e+00
0.000000e+00
0.000000e+00
1
PSAD56
6.378388e+06
3.367003e-03
-2.700000e+02
1.880000e+02
-3.880000e+02
1
Hayford
6.378388e+06
3.367003e-03
0.000000e+00
0.000000e+00
0.000000e+00
1 Clarke 6.378206e+06 3.390060e-03 0.000000e+00 0.000000e+00 0.000000e+00
It's convenient to notice that translations must be carefully inserted allow SPRING to correctly chage from an Datum to another. For this, it1s important to use official parameters calculated by each country official Cartography and Geodesy agencies. Alterations done for one project may be repeated to others, changing the identifiers.
See also:
Projections