If you are using a file of predefined values, you can choose between an ASCII file or a BIN file.
ASCII File Shift
The ASCII file option uses a file containing points that represent a boundary around the entire surface, or the section of the surface you wish to shift. Using this boundary, a TIN will be calculated according to the values in the ASCII file. These values will then be applied to the values in the area of the surface encompassed by the boundary.
If using an ASCII datum model file, the file must follow a specific format:
• The data must be in comma delimited format.
• The coordinate type must be Geographic and the format must be decimal degrees (Latitude, Longitude, Z).
• The hyphen symbol (-) must be used to designate coordinates in the Southern or Western hemispheres.
An example of an ASCII shift file is shown below:
-33.848326,151.192435,10.4
-33.849484,151.192370,10.1
-33.849237,151.193173,9.7
-33.849506,151.194227,9.8
-33.848298,151.193916,10.0
-33.848402,151.193149,10.2
Values in the ASCII file are always Z positive down by design. If your ASCII file contains negative values, this will be reflected in the resulting shift values. For example, a file with a value of -10 applied to a surface that is positive down with a depth of 20m, would result in a depth of 10m. If you were to apply that same file to a surface that is positive up, the result would be a height of 30m.
Using the example above, the following table shows what the resulting values would be with a given set of original values:
Original Value | Shift Value | Resulting Value |
8.2 | 10.4 | 18.6 |
8.6 | 10.1 | 18.7 |
7.3 | 9.7 | 17.0 |
7.4 | 9.8 | 17.1 |
9.1 | 10.0 | 19.1 |
9.2 | 10.2 | 19.4 |
BIN File Shift
The BIN format option used for vertical datum transformations is based on the US NGS format for GEOID99, published here:
http://www.ngs.noaa.gov/GEOID/GEOID99/.
The BIN format is described here in the Frequently Asked Questions section of the GEOID99 web site:
http://www.ngs.noaa.gov/GEOID/USGG2009/faq_2009.shtml.
A BIN file consists of a 44-byte header followed by “nla” rows of data, each row being “nlo” elements long and each element being a 4-byte floating point number. This format is known in FORTRAN lingo as “direct access binary”.
The exact ordering of the bytes is shown below:1
Bytes | Data Type | Variab Name | Variable Description |
1-8 | real*8 | glamn | Southernmost Latitude of grid (decimal degrees) |
9-16 | real*8 | glomn | Westernmost Longitude of grid (decimal degrees) |
17-24 | real*8 | dla | Latitude spacing of grid (decimal degrees) |
25-32 | real*8 | dlo | Longitude spacing of grid (decimal degrees) |
33-36 | int*4 | nla | Number of rows in grid |
37-40 | int*4 | nlo | Number of columns in grid |
41-44 | int*4 | ikind | Set to "1", meaning the gridded data is "real*4" |
|
|
|
|
45-48 | real*4 | data (1,1) | Gridded value at element 1,1 (Southwest corner) |
. . . |
|
|
|
The remainder of the file continues as 4-byte real values. The south row is filled in first, with data(1,nlo) being the last variable in the south row, and then proceeds northward for each consecutive row.
Note: Values in a BIN file are always Z positive up by design. |
The total number of bytes in a BIN file is found using the formula: 44 + 4*nla*nlo.
The actual numbers of rows (nla) and columns (nlo) for each file is the same within a region, but varies between regions, as shown below:2
REGION | ROWS (nla) | COLUMNS (nlo) |
Conterminous United States | 1081 | 1141 |
Alaska | 721 | 1921 |
Hawaii | 361 | 421 |
Puerto Rico and the Virgin Islands | 361 | 301 |
To apply a vertical shift using an ASCII or BIN file:
1. Select a surface root layer in the Layers window.
2. Select the Vertical Shift command.
Menu | Tools > Surfaces > Vertical Shift |
Pop-up | Vertical Shift |
The Vertical Shift Surface dialog box is displayed.
In this dialog box, you define the attribute layers you want shifted and the amount by which to shift them. The resulting surface will contain all of the same layers as the source surface, but only those selected for shift will have their values shifted.
Any layer with a Z value can be shifted.
If the source surface contains a computed layer, that layer will lose its dynamic status and will no longer be affected by changes to the primary elevation layer. |
3. To select a layer, click the layer name in the Available list, then click the Add (right-arrow) button to move it into the Selected list.
4. [Optional] Repeat step 3 for each layer you want to add.
5. [Optional] You can also remove layers from the Selected list by clicking the layer name in the list, then clicking the Remove (left-arrow) button to return the layer to the Available list.
The primary elevation layer is required and cannot be removed from the Selected list. |
6. Select either ASCII file or BIN file from the Shift type drop-down list, depending on the type of file being used.
7. Click the Input file Browse (...) button and select an available file for the selected shift type and click Open.
The selected file is displayed in the Input file field.
You have the option of selecting the vertical reference system that will appear in the CSAR metadata for the resulting surface created for the shift. The options available in the list are controlled by the Vertical Reference System database. This list can be edited using the Reference System Editor tool. For more information, see Vertical Reference System.
8. Select a Vertical Datum option from the list.
9. To choose the name and location of the resulting surface, click the Output surface Browse (...) button.
A Save As dialog box is displayed.
10. Select a path and type a name for the shifted surface, then click Save.
11. To finish, click OK.
A new surface is created and displayed in the Layers window.
The attributes and properties from the source surface are carried over to the shifted surface. Additional properties are also populated under the Sources group.
• Operation: This property identifies the tool that was used to create the surface. For a shift, it will read “Vertical Shift”.
• Vertical Coordinate System: The vertical coordinate system that was used to create the shifted surface.
• Additional Attributes: This property identifies the attributes from the source surface that were shifted.
• Shift Method: This property identifies the type of shift that was applied to the surface.
• Shift File: This property differs depending on the type of shift applied. For ASCII and BIN shifts, it identifies the file of depth values that was used to shift the surface.
• Original Depth Update: This property states whether or not the original depth value of any designated soundings was also updated by the shift. This property is only present if the source surface contained designated soundings.
• Source: This property identifies the name and location of the source surface.
The Z values in the selected layers should be shifted according to the selected method. To confirm that the shift was successful, compare the Z values at various coordinates in the new surface to the values in the selected datum file. The elevation values should match.
1 Use of material from NOAA’s National Geodetic Survey (NGS) is for descriptive purposes only and does not imply endorsement by NGS of CARIS products or services.
2 Use of material from NOAA’s National Geodetic Survey (NGS) is for descriptive purposes only and does not imply endorsement by NGS of CARIS products or services.