Vertical resolution

[turman]

Hi,

What is the resolution of SRTM dataset? Currently srtm30plus is used in WWJ SDK. Does the 30 prefix stands for 30m vertical resolution? And what is its "plannar" resolution?

Looking at Wikipedia, it seems that it's one degree second for USA and 3 seconds for the rest of the world. Hmm one second is around 30m isn't it? So i'm not sure 30 in srtm30plus really stands for the vertical resolution..

BTW if you want to play with VerticalExaggeration add this snippet to BasicDemo.AppFrame.getContentPane():

                        JPanel confPanel = new JPanel(new GridBagLayout());
                        {
                            JLabel l = new JLabel("Vertical Exaggeration: ");
                            confPanel.add(l);
                            SpinnerNumberModel m = new SpinnerNumberModel(1d, 0d, 10d, 0.1d);
                            JSpinner s = new JSpinner(m);
                            class VertExaggerationListener implements ChangeListener {
                                public void stateChanged(ChangeEvent evt) {
                                    JSpinner spinner = (JSpinner)evt.getSource();
                                    Double newValue = (Double) spinner.getValue();
                                    Double oldValue = (Double) spinner.getPreviousValue();
                                    Model m = wwd.getModel();
                                    SceneController sc = wwd.getSceneController();
                                    if (sc != null) {
                                        wwd.getSceneController().setVerticalExaggeration(newValue);
                                        //sc.firePropertyChange(AVKey.VERTICAL_EXAGGERATION, oldValue, newValue);
                                    }
                                }
                            }
                            s.addChangeListener(new VertExaggerationListener());
                            s.setValue(1.0);
                            confPanel.add(s);
                        }
                        westPanel.add(confPanel);

It's so nice to play in Java with WW!

[patmurris]

As you found out, 30m is the distance between two samples on the ground.

However, i think only the US has such a resolution, the rest of the world is mainly 90m - for land, but additional levels have been 'computed' to fill the gap to the 30m level. In many places you can see 'ripples' in the lower levels, which are probably artefacts of this process.

[turman]

Thanks pat!

And do you know the vertical resolution?

[withak]

The .bil files hold signed 16-bit integers, so it can't be less than one meter. I'm not sure what the vertical resolution of the actual source data is. I doubt it's less than one meter.

[TomServo]

You can get down to 10 meter for some areas. Would be nice to start migrating some of that dem data into WW's servers.

[KoS]

Got to NASA's website and look for the metadata. It will tell you the vertical resolution.



KoS

[tag]

WWJ uses:
* SRTM30Plus (global bathymetry)
* SRTM 3-arc/sec v2
* USGS NED 1-arc/sec

Here's the readme from the source of srtm30plus:
SRTM30_PLUS: DATA FUSION OF SRTM LAND TOPOGRAPHY WITH MEASURED AND ESTIMATED SEAFLOOR TOPOGRAPHY

SRTM30_PLUS V2.0 July 29, 2006
Joseph J. Becker jjbecker@ucsd.edu
David T. Sandwell

________________________________________ ________________________________________ _________
Version 2.0 the identical format as V1.0. There are two main improvements:

1) The land datacome from version 2 of the SRTM30 processing. Here is the
information on SRTM V2 provided by Tom Farr of JPL on October 10, 2005.

"NASA has released version 2 of the Shuttle Radar Topography Mission digital topographic
data (also known as the "finished" version). Version 2 is the result of a substantial
editing effort by the National Geospatial Intelligence Agency and exhibits well-defined
water bodies and coastlines and the absence of spikes and wells (single pixel errors),
although some areas of missing data ('voids') are still present. The Version 2 directory
also contains the vector coastline mask derived by NGA during the editing, called the
SRTM Water Body Data (SWBD), in ESRI Shapefile format.

The data may be obtained by anonymous ftp to: e0srp01u.ecs.nasa.gov and moving to the
directory srtm where both version 1 and version 2 directories may be found. Please read
the appropriate documentation, also found in the directories."

2) We have added several new bathymetry grids derived from large multibeam
surveys. The new data include a large IFREMER survey of the Southwest Indian Ridge
provided by Mathilde Cannat, a compilation of the Lau Basin provided by Brian Taylor of
SOEST, and a large survey of the Foundation Seamounts provided bu Marcia Maia at IFREMER.


SRTM30_PLUS V1.0 November 11, 2004
________________________________________ ________________________________________ _________
INTRODUCTION
This data consists of 33 files of global topography in the same format as the SRTM30 products
distributed by the USGS EROS data center. The grid resolution is 30 seconds which is roughly
one kilometer.Land data are based on the 1-km averages of tropography derived from the
USGS SRTM30 gridded DEM data product created with data from the NASA Shuttle Radar Topography
Mission. GTOPO30 data are used for high latitudes where SRTM data are not available. Ocean data
are based on the Smith and Sandwell global 2-minute grid between latitudes +/- 72 degrees.
Higher resolution grids have been added from the LDEO Ridge Multibeam Synthesis Project and the
NGDC Coastal Multibeam Data. Arctic bathymetry is from the International Bathymetric Chart of
the Oceans (IBCAO) [Jakobsson et al., 2003]. All data are derived from public domain sources
and these data are also in the public domain.The pixel-registered data are stored in 33 files
with names corresponding to the upper left corner of the array shown below.

The USGS SRTM30 data and documentation is available at
ftp://edcsgs9.cr.usgs.gov/pub/data/srtm/SRTM30

The US continental coastal multibeam data is available at
www.ngdc.noaa.gov/mgg/coastal/coastal.html
Data from Puerto Rico and Hawaii is -NOT- presently included.

The ocean ridge multibeam data is available at
http://www.ocean-ridge.ldeo.columbia...html/home.html

The artic ocean bathymetry is from
http://www.ngdc.noaa.gov/mgg/bathyme...ic/arctic.html

The Antarctica data (starting at 72S) is not very
interesting; the terrestrial data is from GTOPO30,
and the limited bathymetry is from JEBCO.

________________________________________ ________________________________________ _________
DATA FORMATS
Data are provided as binary integers in exactly the same format
as SRTM30. The files must be uncompressed with gzip and are 16-bit
big endian byte order.

These files can be used directly by ER_Mapper software using the header files
stored in the subdirectory ermapper_headers.

These files can be converted to Generic Mapping Tools (grd-netcdf) format
using the scripts in the subdirectory grd.
________________________________________ ________________________________________ _________

[turman]

This is not very clear for me, apparently the question is not very trivial. Tom says it's >10m which sounds a little disappointing for me (but I known it's not your fault Tom! ). And the readme posted by tag doesn't speak about vertical resolution or my english is really too bad?

Originally Posted by withak

The .bil files hold signed 16-bit integers, so it can't be less than one meter. I'm not sure what the vertical resolution of the actual source data is. I doubt it's less than one meter.

I can't understand why a 16 bit integer couldn't have a precision better than one meter, particularly if you consider that STRM doesn't handle very high altitude area (I read somewhere it was limited to ~7000m).

[withak]

Originally Posted by turman

I can't understand why a 16 bit integer couldn't have a precision better than one meter, particularly if you consider that STRM doesn't handle very high altitude area (I read somewhere it was limited to ~7000m).

It's because the elevation values are stored unadjusted so, as I understand it, you couldn't put an elevation value of 100.5 (for example) in the bil files that WW reads. It would have to be 100 or 101. On the other hand, I could be wrong. It wouldn't be the first time.

[tag]

Elevations indeed are given as integers in meters.

Here's info on vertical resolution:

Vertical resolution of SRTM:
Note first paragraph: http://rst.gsfc.nasa.gov/Sect11/Sect11_10.html
"...vertical resolution...varies from 16 to 10 meters..."

Vertical accuracy of NED:
http://seamless.usgs.gov/website/sea...faq.asp#twenty
"The vertical accuracy is basically +/- 7 to 15 meters. It alls depends on
the original source DEM and if it was level 1, level 2, or 10m resolution."

Please don't ask me about the level 1, level 2 or 10m thing, unless you
really have to.