Written by: Anthony Campbell (Graduate Student)
The need to convert the vertical datum of data is a problem particularly prevalent in coastal applications. For example bathymetric data is often in one vertical datum and terrestrial elevations in another. You open the data in ArcGIS hoping on the fly projection takes care of things, only to realize on the fly projection only works for the horizontal datum. This blog post guides the user through the steps necessary to convert LAS data from NAVD 88 to MHW, the methods could also be used for other vertical datum transformations.
Download VDatum software:
VDatum is a tool developed by the National Oceanic and Atmospheric Administration (NOAA) and the National Geodetic Survey (NGS). This tool is designed to convert LAS, ASCII, ASCII Raster or a single point into a variety of vertical datums. VDatum can be downloaded from http://vdatum.noaa.gov/. You will also need tidal transformations, which are regional grids that NOAA has modeled tidal elevations for. If you know you will be working in a particular region you can download only that transformation. The simplest approach is to download the vdatum_all.zip; this includes all tidal transformations for the United States. On the website each transformation grid can be viewed by full name and hovered over to view the geographic extent of the grid to find the one appropriate for your data.
VDatum requires the Java Runtime Environment; if you do not have this software installed on your workstation; links to the software are available on the VDatum website.
LiDAR point data can be acquired for free from the NOAA Digital Coast Data Access Viewer. The blog post “Obtaining Elevation Data from NOAA Digital Coast” provides step by step directions.
If you followed the workflow above, your data is in North American Vertical Datum 1988 (NAVD88). This data needs to be converted from NAVD88 to Mean High Water (MHW); VDatum will be used to accomplish this. A Mean High Water Datum is limited to nearby areas with similar ocean characteristics, by modeling these characteristics for regions NOAA has created transformations grids that allow for more accurate conversion to tidal datums. The MHW elevation will always be zero when projected into the MHW datum.
Examine Transformation Grid:
When utilizing VDatum for coastal conversions it is important to examine the transformation grid for your study area. This can be done by opening ArcMap and navigating to your VDatum folder from within the ArcCatalog window. Within the Vdatum folder you can find all the transformation grid folders.
Once you have unzipped the VDatum_all.zip file into a folder, you will be able to view all of the transformation grid folders and core files necessary to run VDatum.
There are a few ways in which you can run VDatum.
- First, you can run VDatum from the command line; a guide for this method is available on the VDatum website.
- Second, you can open VDatum’s graphical user interface (GUI) from the Windows Batch File, or from the Executable Jar File which can be found within the VDatum folder below the transformation grid folders.
The Jar file is the quickest way to access the GUI for VDatum, so it is recommended to open VDaturm in this way. Below you will find screen captures of the contents of your VDatum folder will look; the Windows Batch File, and the Executable Jar File are shown in red boxes.
In this figure, you can see the transformation grid overtop the 2013 National Agriculture Inventory Program imagery, for the Sandy Hook Unit of Gateway National Recreation Area. The transformation grid covers the majority of the landform, but does lack coverage towards the middle of the peninsula (callout map). This area of sparse grid coverage should be taken into consideration when analyzing any resulting data products. Next we will discuss the process of converting our LAS files.
- Open VDatum, Click the file conversion tab. Next select the files by clicking the button to the far right of File name(s).
- The Open file(s)… dialog box appears. A good shortcut to finding and adding your LAS data is to use *las to filter your files.
- After the file(s) have been selected. The horizontal and vertical Source parameters will be automatically completed.
- Next, the Target parameters need to be filled out. First make sure the horizontal data target parameters match your source inputs. For the purpose of shoreline derivation MHW is the preferred datum and should be selected as the target vertical datum, target unit is meter (m).
- Check the box Excluding NODATA points (points with coors. = -999999). This will facilitate the assessment of VDatum’s operational transformation grid extent for your study area. Click the convert button to begin the conversion process.
- Click the convert button to begin converting LAS data in NAVD 1988 into MHW, a converted set of LAS files will be written into your target folder.
- The output file defaults to creating a new folder within the same folder as your target LAS files called result, where converted LAS files will be written.
The data are now in MHW vertical datum, meaning a zero value for elevation corresponds to MHW as determined by NOAA. The data can be evaluated within ArcMap by creating a LAS dataset with the LAS files; next the point distribution can be examined. The example below shows the original point coverage, and the coverage following the conversion with VDatum. For delineating the ocean side shoreline the converted points have adequate coverage. ASCII files containing elevation data can also be converted using VDatum. The converted data can be utilized for a variety of purposes such as delineating MHW shorelines or determining dune elevation above MHW.
This blog posting was developed with the support of a competitive grant (cooperative agreement number P09AC00212; task agreement number P13AC00875) from the National Park Service in partnership with the North Atlantic Coast Cooperative Ecosystems Studies Unit.