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=Introduction=
High-Resolution (HR) Grid/Raster Map Outputs was first introduced in TUFLOW 2020-10-AB release. When an SGS model uses “<font color="blue"><tt>SGS Approach </tt></font> <font color="red"><tt>== </tt></font> <font color="black"><tt>Method C</tt></font>”, the sampled elevations are retained at the end of the geometry processing. These sub-grid elevations include topography modifiers such as breaklines, and they allow a high-resolution elevation check file to be written and used for high-resolution depth map outputs. Currently, ASC, FLT and TIF raster formats are supported.
 
Example models with high-resolution outputs are available in the <u>[[TUFLOW_Example_Models|TUFLOW Example Models]]</u> dataset.
For the HR output, the water level at each HR output location is interpolated from the computed 2D water levels. The depth is the difference between the interpolated water level and the sub-grid elevation. This differs from the standard depth output which calculates the depths at cell centres and corners first, then interpolates the depths to the standard output grid locations (default is half the cell size). The difference between the interpolation processes is illustrated below.
 
For the HR output, the water level at each HR output location is interpolated from the computed 2D water levels (see <u>[[#Water_Level_Interpolation_Methods|Water Level Interpolation Methods]]</u> for details). The depth output is calculated differently for standard and high-resolution outputs:
:* Standard depth output: The depth is calculated at cell centres and corners first, then interpolated to the standard output grid locations (default is half the cell size). This is shown in the image on the left.
:* High-resolution depth output: The depth is the difference between the interpolated water level and the sub-grid elevation. This is shown in the image on the right.<br>
<br>
[[File:Sgs_std_output.png|360px]] [[File:Sgs_hr_output.png|360px]]
 
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=Water Level Interpolation Methods=
Similar to the standard output, the HR output needs to interpolate cell centre water levels to cell corners. However, the interpolation methods for the standard output (<font color="blue"><tt>Map Output Corner Interpolation </tt></font> <font color="red"><tt>== </tt></font> <font color="black"><tt>Method C</tt></font>) can produce "bumpy" HR water level outputoutputs in direct rainfall models with steep terrain, as the water level is linearly interpolated from the cell centres/corners, while the change of sub-grid elevations may not be linear. This often happens between fully wet cells and sheet flow cells, as illustrated below.
 
 
[[File:corner_h_intp_output.png|480px]]
 
 
In the HR water level output, these locations often have high water level with triangular shape.
 
 
[[File:HR_Intp_A.png|480px]]<br>
'''Method A HR Interpolation Approach == Method A''' (Graygray lines show the triangulation TINs)
 
 
Two additional methods have been added for the HR corner water level interpolation:
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* Method C is the default option that applies the same sheet flow checks as the Method B. In addition, it also uses the number of wet SGS sampled points as a weighting that biases non-sheet flow cells that further improves the mapping to in-stream water levels.
 
The two images below present the high-resolution water level output at the same location, but with <tt><font color=blue>HR Interpolation Approach </font><font color=red>==</font> Method B</tt> and Method C<tt>MethodC</tt>. As can be seen, the water level along the narrow stream is “smoother”.
 
 
[[File:HR_Intp_B.png|480px]] [[File:HR_Intp_C.png|480px]]<br>
'''HR Interpolation Approach == Method B (left) and Method C (right) HR Interpolation Approach'''
 
Note that when these two methods are applied, the interpolated corner water level is biased to the non-sheet flow cells, and consequently, sheet flow cells may appear as “dry” cells. The improved approach that takes into the account of the sheet flow water level is currently under development. ▼
 
▲Note that when these two methods are applied, the interpolated corner water level is biased to the non-sheet flow cells, and consequently, sheet flow cells may appear as “dry” cells. The improved approach that takes into the account ofthat the sheet flow water level is currently under development.
Whilst Methods B and C can substantially improve the water surface mapping of SGS models using direct rainfall (rain-on-grid), there will always be inaccuracies with mapping at a higher resolution than the 2D cell resolution due to interpolation and extrapolation. Regardless of the software, the greater the ratio of 2D cell size to the high-resolution DEM cell size, the greater the potential for mapping inaccuracies. Should better mapping accuracy be required, reducing the 2D cell size to compute the spatial variation in water surface and velocities more accurately is, by far, the best course of action. ▼
 
▲Whilst Methods B and C can substantially improve the water surface mapping of SGS models using direct rainfall (rain-on-grid), there will always be inaccuracies with mapping at a higher resolution than the 2D cell resolution due to interpolation and extrapolation. Regardless of the software, the greater the ratio of 2D cell size to the high-resolution DEM cell size, the greater the potential for mapping inaccuracies. Should better mapping accuracy be required, reducing the 2D cell size to compute the spatial variation in water surface and velocities more accurately is, by far, the best course of action.
Please also note that it is not necessary to use these options for non-rainfall on grid models.
 
=Interpolation near Thin BreaklineBreaklines =
Thin breaklines are often used to define hydraulic controls, such as levees and road embankments. These controls often experience upstream controlled weir flow, where the water levels on the two sides of the breakline can be considerably different. Retaining just one water level (upstream, downstream or average) at output points along the breakline may result in unsatisfactory water level map output as illustrated in the two figures below. This issue becomes more profound the larger the 2D cell size.
: '''Note:''' thisThis feature (at present) only works for thin breaklines in Z Shape (2d_zsh) layers. Thick/wide breaklines raise more than 1 rowsrow of cell centres and 2 rows of cell faces, and thus do not experience similarthis HR interpolation issue. ▼
 
[[File:breakline_h_intp_output.png|480px]]<br>
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'''HR Thin Z Line Output Adjustment == OFF'''
 
A new HR output feature has been introduced to improve the water level interpolation along thin breaklinebreaklines.
AnyFor any further questions please email TUFLOW support: [mailto:support@tuflow.com?Subject=TUFLOW%201D%20gates%20help support@tuflow.com]
 
<font color="blue"><tt>HR Thin Z Line Output Adjustment </tt></font> <font color="red"><tt>== </tt></font> <font color="black"><tt>OFF | {ON CELL SIDES} | ON ALIGNMENT</tt></font>
 
* “OFF”OFF: does not apply any water level adjustment.
* “ONON CELL SIDES”SIDES (default): two water levels, rather than one, are stored at cell corners selected by thin breaklines. These water levels are used to interpolate HR water levels on either side of thin breaklines. This option produces a sudden drop in water level across the breakline as illustrated in the figure below, noting the sudden drop follows the 2D cell faces where the breakline is applied.
* “ONON ALIGNMENT”ALIGNMENT: As an extension to the “ON CELL SIDES” option, this method moves and aligns the 2D cell corner vertices to the original alignment of the breakline for producing HR water level and depth. The arrows in the figure below show how vertices of the TIN are moved to align with the breakline. This can produce substantially improved high-resolution mapping along thin breaklines with substantial drops in water levels, especially for larger 2D cell sizes. Note that the cell corners are NOT shifted for the hydraulic calculations.
 
[[File:HR_thin_brk_on.png|480px]] [[File:HR_thin_brk_on_adj.png|480px]]<br>
'''HR Thin Z Line Output Adjustment == ON CELL SIDES (left) and ON ALIGNMENT (right)'''
 
▲Note: this feature (at present) only works for thin breaklines in Z Shape (2d_zsh) layers. Thick/wide breaklines raise more than 1 rows of cell centres and 2 rows of cell faces, and thus do not experience similar HR interpolation issue.
 
=Showing Face Elevations in _DEM_Z_HR Check File =
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<font color="blue"><tt>HR Grid Output Use Face Elevations </tt></font> <font color="red"><tt>== </tt></font> <font color="black"><tt>{ON} | OFF</tt></font>
 
* “ON”ON (default): useUse face elevations in [[Check_Files_2d_DEM_Z_HR | _DEM_Z_HR]] check file. This option accurately shows the cell faces where water can be hydraulically blocked. However, when cell faces are angled with the thin breakline, this can create 'pockets' of lower elevations in the check file that do not exist in reality.
* “OFF”OFF: Face elevations are not used in [[Check_Files_2d_DEM_Z_HR | _DEM_Z_HR]] check file. This is an useful option for report writing, as 'clean' high-resolution DEM images can be generated reflecting all geometry updates except for the Thin Breakline updates at cell faces.
 
[[File:HR_Z_Use_Face_Elevations_ON.PNG|480px]] [[File:HR_Z_Use_Face_Elevations_OFF.PNG|480px]]<br>
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