:: Chapter 02 - AsterDTM Operation
AsterDTM is an ENVI plug-in for extracting digital elevation values from ASTER 1A and 1B images. Please refer to chapter 2 "Installing AsterDTM" for instructions how to install AsterDTM.
The ASTER (Advanced Spaceborne Thermal Emission and Reflection Radiometer) sensor is a cooperative effort between NASA and Japan's Ministry of Economy, Trade and Industry (METI) and the Earth Remote Sensing Data Analysis Center (ERSDAC). Flying on the TERRA platform, a satellite launched in December 1999 as part of NASA's Earth Observing System (http://ASTERweb.jpl.nasa.gov/), ASTER covers a wide spectral region with 14 bands from the visible to the thermal infrared with high spatial, spectral and radiometric resolution. The spatial resolution varies with wavelength: 15 m in the visible and near infrared (VNIR - 0.55 to 0.80um), 30 m in the short wave infrared (SWIR - 1.65 to 2.4um), and 90 m in the thermal infrared (TIR - 8.3 to 11.32um). An additional band (named 3B, for "backwards") works at the same spatial resolution as the ordinary band 3 (named 3N, for "nadir") of the VNIR sensor, but at a backward angle of approximately 28 degrees, producing a stereo pair for each ASTER image.
The basic principle behind the DTM extraction with AsterDTM is the well known parallax effect - you look at an object from two different angles and thus can obtain it's third dimension. Each ASTER image contains it's own stereo pair - looking at the same terrain from two different angles, provided in the form of a 3N / 3B stereo pair with 15 m spatial resolution. AsterDTM converts these two bands into a pair of quasi-epipolar images, which have a pixel displacement in the satellite flight direction proportional to the pixel elevation. A cross-correlation method is used to determine this displacement, which in turn is transformed into elevation values.
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Fig.01: AsterDTM Start Center
To start AsterDTM, navigate within the ENVI main menu to the menu entry for the AsterDTM module and click on it. The AsterDTM command center pops up, with initially most processing options grayed out.
These options will be made available as the processing of your ASTER image proceeds. The status line on the bottom of the command center describes the current action, or the processing step to follow.
As of AsterDTM version 2.0, a short cut menu is available with a subset of the processing options, accessible by clicking with the right mouse button anywhere in the background of the AsterDTM start window:
Please see the chapter "Options menu -> Defining processing options" for a detailed description of theses processing options.
The command center has two pulldown menus, "File" and "Options". Click on "File -> Open ASTER HDF file (1A or 1B)", or click on the "Aster Input File:" button to open an ASTER HDF 1A or 1B file through the standard ENVI file selection dialog (fig. 2). If the ASTER file you want to process is already open then you can directly select it from the list, otherwise click in "Open File…" to open the file selection window and navigate to the file you want to open (Observation: the usual option to spatially subset the image "Spatial Subset" is not made available here because the subsetting is being done in a latter stage).
Fig.02: File selection dialog windows
When you open an ASTER file in HDF format in ENVI, then this file is split up into its image data sets, depending whether it's level 1A or 1B. The ASTER 1A HDF files are composed of six image files: the three VNIR (15 m) bands separated (Band1, Band2 and Band3N), the backwards 3B band (band3B), the six SWIR bands together, and the five TIR bands together. The ASTER 1B HDF files are composed of 4 image files: the three VNIR bands together, the backwards 3B band (band3B), the six SWIR bands together, and the five TIR bands together.
You may select any of these data sets, as AsterDTM automatically identifies the VNIR components. If the file is not a valid ASTER 1A or 1B HDF file, an error message is displayed.
After having selected the ASTER input file, a status window "Reading ASTER data sets…" pops up, and the image files, geometric and radiometric calibration data and all other necessary information are extracted from your ASTER file. The subsequent processing steps depend on whether the selected image is of type level 1A or level 1B.
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VNIR:
The level 1A VNIR bands (including band 3B) have to be corrected radiometrically and geometrically in order to be able to extract elevation values through parallax evaluation. As a first step, the user is prompted for an output filename for the 1A-1B VNIR corrected file:
A suggested output file name is automatically constructed from the input ASTER file name. It can be edited directly, or the "choose" button can be used to navigate to the desired output directory and file name.
If the corrected ASTER 1A->1B VNIR file for this particular ASTER image has already been produced before, the latter file dialog will be replaced by the one below. In case the selected file name points to an already existing file, then this file is being examined internally in order to determine whether it is the valid result of a previous 1A->1B conversion; if this is the case, the following dialog window pops up:
The first option, "Use already existing file", opens the selected file read only and jumps directly to the warping of band 3B, and the second option overwrites the already existing file. The third option "Select a different output file name below" is effectively there only to complete the list of options, and is executed through selecting a new file name with the "choose" button.
In case the selected file name already exists but is not identified as the corresponding 1A->1B file, the user is prompted whether he really wants to overwrite this file.
The processing of the ASTER 1A bands goes through the steps of replacing bad lines, applying radiometric calibration coefficients in order to remove banding and striping effects, removing any remaining high frequency noise from the 3N band, geometrically correcting bands 1,2 and 3N, and finally co-registering these bands with respect to each other. The same processing is then applied to the 3B band, creating a temporary file in the temporary directory defined by the ENVI users preference "Default Temp Directory".
SWIR / TIR:
ASTER 1A SWIR and TIR bands can be corrected geometrically and radiometrically through the options
"File" -> Create 1A->1B SWIR equivalent" and
"File" -> Create 1A->1B TIR equivalent"
in the start center File menu. The dialog window is similar for the SWIR and TIR bands (except that for TIR the filter option is not available, because there is no need for filtering), and is shown below for the "File" -> Create 1A->1B SWIR equivalent" case:
The user has the option to select whether or not filtering (FFT filter) and/or radiometric correction is to applied, and whether the output is to memory or to a disk file. In case the disk file already exists and carries the specifics of the SWIR 1A_>1B corrected file, the user is prompted to either use this existing one, overwrite it, or choose a new output filename, very much like in the VNIR radiometric and geometric correction.
Geometric correction of SWIR and TIR 1A bands is achieved by automatic identification of tie points between the SWIR/TIR bands and VNIR band 3N, on a band-by-band basis , and subsequent registering. The success of this operation depends on the quality of the image (low cloud coverage) and sufficient areas of high correlation. It is recommended to check the quality of the inter-band registration for each instrument, prior to further processing of these results.
Once the operation has been completed, a check mark appears at the corresponding menu entry to signal the execution of this task.
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Defining DTM output file name and Spatial Subset
Having selected an ASTER 1B file, or at the end of the pre-processing of an ASTER 1A file, the user is prompted to define the spatial subset for the processing, and the output file name for the DTM to be extracted:
The "Output Result to" option lets you choose between saving the resulting DTM to the file defined under "Enter Output Filename", or to system memory. The button "Spatial Subset" opens the ENVI standard dialog window for spatial sub setting. Please see "Options Menu: Defining Processing Parameters" for the option to output the resulting DTM north oriented, instead of the default orbit oriented.
Using Ground Control Points (optional)
One of the strengths of the AsterDTM module is the capability to extract and use the ground control points provided by the ASTER image itself, stored in the ephemeris part of the HDF file. The precision of these internal GCP's is extraordinarily high, allowing the geopositioning of the orthocorrected bands with an x/y error of less then 50 m, without the need of any external ground control points (GCP's).
However, AsterDTM - starting from version 2.0 - provides the option to import and collect GCP's in order to either improve the georeferencing from ephemeris data and/or calibrate the elevation values to turn the relative output DTM into an absolute one. GCP's can be entered either in the form of a pre-existing GCP points file (ENVI default *.pts, through the "GCP Input file:" button), or by starting the collection of GCP's through the "collect GCP's" button in the start window.
GCP
Input File:
Use this option to load a GCP file resulting
from the previous collection of ground control points in ENVI (either
directly, or within the AsterDTM module). There are two types of files
which can be imported:
· The GCP file produced through the "File->Save GCPs w/ map cords …" option in the Ground Control Points Collection windows
· The GCP list (table) produced through the "File->Save table to ASCII …" option in the Ground Control Points List window (you open this window in the Ground Control Point Collection window by clicking on "Show List")
Either of these files can be the result of a previous GCP collection from within AsterDTM , or from a GCP collection within standard ENVI. The main difference between the two files is that the GCP file contains all collected points, not indicating which files are "On" and which are "Off", whereas the GCP list indicates which points have currently been disabled for processing by the user (please see the standard ENVI documentation Map Tools->Rectification and Map Tools->Orthorectification for a detailed description of the GCP collection window and it's processing options).
Collect GCP's
Use this option to interactively collect ground control points from within the AsterDTM module. Selecting this option will automatically open a new display window with the ASTER 3N band displayed (grey scale), and the standard ENVI Ground Control Points Collection window (with the option of entering elevation values).
GCP's
are being collected by clicking on the selected location either in the
image window or in the zoom window (recommended), typing in the corresponding
elevation value (in meters), and the geographic coordinates. Please mind
that the map projection is automatically set to the projection of the
input ASTER image, and must not be modified. Geographic coordinated may
be imported from other images or vectors, regardless of their map projection,
and will be converted on the fly by ENVI (f.i. open the georeferenced
base image in another display, open the pixel locator with "tools->pixel
locator", in the pixel locator window click on "options->
Map coordinates", left-click on the desired position in the base
image (in the zoom window), and click on the Export" button - the
coordinates will be converted on the fly and inserted in the East and
North fields of your Ground Control Points Collection window (for vectors
there is a similar mechanism - open the vector in a new vector window,
right-click in the vector window and use "Export Map Location").
Add new newly collected point by clicking on the "Add Point"
button in the GCP collection window.
There are three kinds of control points you may define:
- 1. XYZ GCP's: all available information (x, y coordinates and elevation ) is provided by the user. These points will be used for both georeferencing the output images and calibrating the output DTM.
- 2. Horizontal tie points: If you have the x and y coordinates for a specific point but the elevation at this location is unknow, you may use this point as a horizontal tie point (it will be used for the georeferencing process, but not for the calibration of the output DTM). Please enter -999 (or less) in the elevation field to indicate that this is a horizontal tie point.
- 3. Vertical tie points: If you now the elevation at a certain location, but not the map coordinates, you may use this point as a vertical tie point (it will be used for calibrating the output DTM, but not for the georeferencing process). This is particularly useful if you want to fix a location to a certain elevation value . Please enter 0 (zero) in the E and N fields of the map projection information to indicate that this is a vertical tie point.
There
are a lot of options which allow you to handle and manage GCP's more efficiently
- please see the standard ENVI documentation Map Tools->Rectification
and Map Tools->Orthorectification for a detailed description of the
GCP collection window.
The minimum number of GCP's to enter is one; for refining the georeferencing
of the output images a linear regression is being used to define the x/y
offset ("rubber sheeting"), and for the heights information
a linear regression is used as well to define the offset between the absolute
elevation values entered by the user, and the elevations determined by
the AsterDTM module. Please mind that this is a case where the RMS error
is not a good measure for the quality of your ground control points, as
you are setting the points in an image which is yet to be orthocorrected.
This orthocorrecton may imply shifts of up to several hundred meters,
depending on the absolute elevation and the pointing angle of the instrument.
Once
you finished collecting your GCP's, before transferring them to the AsterDTM
module, you should save them to a file either by using the File->Save
GCPs w/ map coord …" option in the GCP collection window, or
through showing the list of GCPs with "Show List" and saving
this list through the "File-> Save Table to ASCII" option
in the GCP List window .
The newly collected GCP's have to be transferred to the AsterDTM module
by means of the "Export points to AsterDTM" entry in the options
menu of the GCP collection window:
Please
mind that this will transfer all collected points, regardless whether
they are switched on or off in the GCP list window.
The GCP information will be silently used to improve the georeferencing
of the output images, and to calibrate the output DTM. You may check the
effect of your input GCP's by disabling the use of the GCP's , by checking
off the "Use GCP's" button (either right-click in the start
center window, or choose "Options->Define processing parameters"->Processing
and check off "use GCP's box).
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Starting the ASTER DTM extraction
Once the DTM output file and the spatial subset have been defined, the start center shows the file definitions, the "Change spatial subset" button and the "Start ASTER DTM extraction buttons are active, and the currently selected spatial subset and the approximate time to extract the DTM for this subset are shown.
The
"DTM Output File" and the "Change spatial subset"
buttons provide the same functionality of defining the output file name
and the spatial subset.
Clicking on "Start ASTER DTM Extraction" starts the process
of iteratively extracting and refining the digital elevation model for
the selected spatial subset.
The extraction of the DTM requires four correlations scans plus post processing, and takes about the time indicated in "Estimated processing time:", estimated on the basis of a single processor/single user configuration. Once the process is finished, the resulting DTM and the corresponding correlation score map (see "Analyzing the correlation score map" for details) show up in ENVI's Available bands list (ABL), and the "Apply orthocorrection" button turns active.
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Once
the DTM extraction process has finished, the produced elevation model
can be used to correct the ASTER 1B bands or the ASTER 1A->1B corrected
bands, for displacement errors caused by the parallax effect, particularly
useful in mountainous regions and for ASTER images with a non-zero pointing
angle (instrument looking aside). The procedure is slightly different
for ASTER 1A and 1B images.
Clicking on the "Apply orthocorrection" button opens a dialog
window similar to the one below. (for ASTER 1A images, if neither for
SWIR nor for TIR the geometric and radiometric correction has been executed,
this dialog window is being skipped) :
After selecting the desired subsystem files (one at a time) , the following dialog window appears,
allowing the definition of the spatial subset and the output file name for the orthocorrected file, as well as an option for defining the output file geographic orientation (orbit oriented, or north oriented). For ASTER 1B images only, there is also an option for calibrating the output DN values to radiance, using the conversion coefficients in the ASTER 1B HDF file (for ASTER 1A images, the calibration - if so desired - has to be executed together with the geometric correction). The spatial subset is effectively limited to the area of overlap between the image and the DTM, and may only be smaller but not larger then the subset initially indicated.
The result of the orthocorrection is accessible through the Available Bands List.
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OPTIONS MENU
Options menu: Display ASTER File header info
This options displays information extracted from the ASTER HDF data entries:
Options menu: Defining processing options
There are several processing and output options the user can change through the "Options -> Define Processing Parameters" menu entry in the AsterDTM start center. The processing parameters window is divided in four sections, each one accessible through clicking on the tab title:
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DEM min/max value: |
Minimum and maximum elevation values to be encountered
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DEM constant offset: |
The extracted DTM is a relative one; this option allows applying
an offset to all elevation values (positive or negative), either
prior to starting the DTM extraction, or after having finished processing.
In the latter case the user is being asked whether he wants to apply
the offset immediately.
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DEM output pixel size: |
The output DTM pixel size in meters. Available pixel sizes are 60m / 30m / 15m . A smaller pixel size takes longer to process (about 3 times), and vice-versa. Important: elevation values are always extracted at the highest possible resolution (15 m), no matter what the setting of the output DEM pixel size is.
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Output DTM north oriented |
Click on the check box to have the output DTM and the correlation image rotated to be north oriented. The default for these images is to be orbit oriented , which results in a rotation angle (relative to the map grid) of around -10 degrees. Choosing this option is particularly useful when you want to mosaic the output DTM's. Please mind that you can use at any time Basic Tools->Rotate/Flip Data from within the standard main ENVI menu to rotate the orbit oriented output images. If this option is chosen, then north orientation of the orthocorrected images is also the default.
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DEM Nodata value: |
The numerical value that AsterDTM assigns for regions where no elevation value could be extracted (f.i. because of lack of image data).
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Interpolation Method: |
The default method for interpolating the correlation values for each image subset is an optimized cubic interpolation along the orbit direction (1-D cubic). A slightly better vertical precision (~5 % improvement) may be achieved by using the 2-D cubic interpolation, although at the price of a significantly enhanced processing time.
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Correlation matrix size: |
In order to identify - on a pixel-per-pixel basis - the correct offset between the nadir band and the backlooking band, it is necessary to compare image chips of a reasonable size - large enough to allow unambiguous identification, but small enough to resolve the morphologic structures (very much like a kernel for convolutive filtering). The default setting is for a 9x9 matrix ; smaller settings result in finer spatial (x/y/z) resolution, but are more susceptible to false correlations, whereas larger values are more robust at a lower resolution.
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Use water detection: |
The
DTM extraction in AsterDTM is fundamentally based on the correlation
between the nadir image and the back-looking image; water, due to
it's homogeneous appearance, is almost impossible to use for a correlation
analysis, rendering very low correlation values and therefore very
noisy, unreal elevation values. AsterDTM , starting with version
2.0, provides an automatic mechanism to identify these water bodies,
and associate to them the border (shore, strand, coastline) elevation.
Water bodies are identified by means of a segmentation of the correlation
image for correlation values less than the correlation threshold
(default: 50 , on a scale from 0 [no correlation] to 100 [identical]),
and for band 3N pixels less than the image threshold (default: 30).
Only segments with more than the minimum population limit (default:
1000) are considered water bodies. To change any of these parameters,
click on "Change parameters", click on the parameter to
modify, and
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Use GCP's: |
Click on the "Use GCP's" box if you want to enable the use of GCP's currently defined for your Aster image. Please see "Using Ground Control Points" for instructions how to define and import GCP's.
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Extended correlation analysis: |
Use this option to enable the extended correlation mode. In the default processing, the last (decisive) processing step is being executed on a relative small image subset around the pixel being processed, with the approximate position of this subset (in other words, the approximate elevation value) being a result of the previous processing steps. If this approximation went wrong, the correlation analysis will fail for this particular pixel, even though eventually the overall conditions for a correlation analysis would be o.k. In the extended correlation analysis mode, all pixels with correlation values less then 70 are being processed once again with a significantly larger image subset (extended range of possible elevations), eventually allowing to identify the correct elevation in another elevation range. However, the use of this option may increase significantly the processing time, and eventually lead to a larger number of false matches (due to the extended elevation range).
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Use external DEM: |
Click the "Use external DEM" box to use an external DEM, either for assisting the elevation extraction, or to orthocorrect the ASTER bands by means of this DEM. In case there is not yet an external DEM defined, a selection dialog will pop up to interactively query the ENVI vector file. Click again to turn off the use of an external DEM.
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Select external DEM: |
Opens the standard ENVI file selection dialog for opening an ENVI file which is to be used as external DEM. Upon importing the file, the area of overlap is determined and an error message is submitted in case there is no overlap between the external DEM and the VNIR 1A->1B file. The external DEM file may be in geographic lat/lon or in any map projection ENVI supports; the units (elevation) are considered to be in meters (please convert units before importing the file if necessary). GTOPO30 files can be opened directly without prior conversion, clicking on the .dem file. SRTM .hgt files are also supported directly. Once an external DEM has been imported successfully, the "Use external DEM" option is checked, and the external DEM will be used in the Aster DEM extraction, unless you uncheck the "Use external DEM" box. Using an external DEM has essentially two effects on the resulting DTM:
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External DEM Nodata value: |
The numerical value for regions in the external DEM where no elevation is defined.
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Use vector layer: |
Click the "Use vector layer" box if you want to use a user-defined vector layer to delimit areas where no elevation extraction should be carried out (please see "Select .evf file" for details). Click again to turn off the use of a vector layer.
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Select .evf file: |
Opens the standard ENVI file selection dialog for opening an ENVI .evf vector file. The use of a vector file is indicated whenever there are medium size or large areas of water (lakes, sea, ocean), which in general produce unreliable results in the correlation process. In case the "Use vector layer" box is checked, AsterDTM takes the following actions on the vector file:
Observation 1: The vector file has to be of the type "polygon", e.g. the result of the vectorization of a classification result.
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Reset: |
Resets all processing parameters to the initial values.
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Accept: |
Saves changes to the settings, and closes the Processing Parameters Definition window.
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Cancel: |
Closes the Processing Parameters Definition window without saving any changes to processing parameters.
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This option displays AsterDTM version information.
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