Follow-up of previous ObsParis / CDS meeting
Initially using Aladin prototype v9.6 (27/6/2017) and TOPCAT v4.4 on MacOS
- See also page: Aladin & planetary surfaces for other use cases / examples.
Partly updated 5/2019 using Aladin prototype v10.1 / based on v10.128 (12/5/2019) and TOPCAT v4.6-3 on MacOS
Set up for planetary mapping
Assuming longitude is E-handed (EPN-TAP convention, and IAU convention in planetocentric frames):
• Standard setup for planetary maps in TOPCAT SkyPlot:
(in Axes / Projection)
Projection = sin (i.e. sphere, for orthographic projection), but car (plate carrée / cylindrical) and Aitoff are also possible (will not rotate)
Uncheck Reflect longitude axis, (keep it checked only if coordinates are provided with W longitudes)
View Sky System: Equatorial,
(in Axis / Grid)
Increase Grid Crowding cursor value to 30 or 60° tick
For cylindrical maps:
(in Axes / Projection)
Projection = car (for 0° at center, range = 0-360°)
Projection = car0 (for 0° on left border)
• Standard setup for planetary maps in TOPCAT PlanePlot (cylindrical):
(in Axes / Range)
Set Max X value to 360
Central meridian will be on left border
• To put the central meridian at the center (range = -180 to 180):
(in Axes / Range)
Min / Max X = -180 / 180
(in Mark / Position)
X: LON > 180 ? Lon - 360 : Lon
(in Axes / Labels)
X Label = Longitude
• From v4.6-3, the SkyPlot window is more versatile for maps (see above)
• Standard setup for planetary maps and HiPS in Aladin:
In the fields on top of the window, set Frame to "Planet" or "Planet deg" - this will use a grid in degrees (counted E and W from 0 - not the IAU convention, but OK)
Set Projection to Sphere (actually: orthographic) or Cartesian (actually: cylindrical) for most applications. Aitoff is ~ similar to sinusoidal, with all the surface visible at once; keep it centered on (0°,0°) !
In properties, .longitude should be set to "ascending" for correct orientation.
Healpix generation from a table in TOPCAT
Healpix is a tesselation system on the sphere, which depends on a resolution/scale parameter. It provides regular sampling in surface, with no singularity (poles).
When using this system, TOPCAT will integrate elements included in the corresponding cells. The benefit is to easily get a combined map from sparse data; the result is also scalable in resolution: at least degradation is performed correctly by combining healpix cells.
Cell size (in FoV size, ie in longitude at Equator): scale 6 ~ 1°, 12 ~ 1', 18 ~ 1"
From the SkyPlot window, several map projections are available with this mode (in Axes panel): "sin" (actually: orthographic, 3D), Aitoff (all surface visible, minimize deformations near the poles), and "car" (cylindrical).
In TOPCAT, load a table containing a list of observations located in lon/lat
(the working example is a large table of VIRTIS/Rosetta observations of 67P)
A detailed step by step tutorial is now available: Use case: mapping sparse spatial data with TOPCAT
1) Standard way (density map from a lon/lat table):
• Open the SkyPlot panel, select table in Position panel, as well as Lon and Lat parameters (C1min and C2min for an EPN-TAP table)
• Go to Form panel, click on the big Forms button (green cross) and "Add SkyDensity" from the local menu. Uncheck the previous Mark Form
• In the Form panel, select parameter to plot as Weight (will be weighted by density), select combine mode, and adjust scale. Healpix generation is performed on the fly, so all scales can be selected.
• Possibly adapt color scale in the Aux Axis tab (left menu)
• In the SkyDensity / Form panel, play with HEALPix level to set a convenient integration scale.
Figure: VIRTIS/Rosetta measurements on 67P, located in lon/lat
• In the Form panel, you can either "Import" to create the integrated HEALPix table in TOPCAT at the current scale (2 columns: cell #, plotting parameter; only filled cells are present), or "Save" in a file in HEALPix-fits format (FITS formatting is more efficient than VOTable, as it is compressed).
• HEALPix-fits maps from TOPCAT (>v4.6-3) open in Aladin and can be, e.g., superposed on a planetary HiPS. Aladin prototype v10.1 (based on v10.127) is required to plot the HEALPix map with correct orientation (set longitude = ascending in Properties panel of the HiPS).
2) Alternatively (from a binned table):
This other method is adapted when you have only 1 or 0 data / healpix cell, ie if the grouping has already been done in a previous step (e.g., with an ADQL query to group data by HEALPix cells, or when opening a previous HEALPix-fits map). It is not intended to group or average table rows in the cells interactively.
• If starting from a lon/lat table, first add a new synthetic column providing healpix cell number (right-click on table column header and enter new column description):
healpixNestIndex( 6, longitude, latitude )
(adapt scale to data in use, 6 provides a resolution of ~ 1° at the Equator)
• (once you have a table with healpix cell # included)
Open Skyplot panel, add New healpix layer, provide inputs
Correct Healpix Data Level in particular (TOPCAT will try to guess, but this is uncertain)
• In Skyplot / Style panel, use Degrade and Combine mode to reduce resolution on the fly (super-resolution can no longer be achieved)
Complex footprints in TOPCAT
This mode allows display of either file footprints (series of images or spectral cubes) or extended pixels footprints (individual spectra from a cube).
Set up is easier if table columns are associated with relevant UCDs (typically pos.bodyrc.lon/lat).
• When FoV have 4 corners (or only 3), use Add New Quadrilateral Plot in a SkyPlot or PlanePlot window
• In the Position tab, set the 4 Lon/Lat pairs to corners values
• In the Form tab, set Mode = Aux and select the adequate parameter as usual
• Set the other graphical parameters as usual
If contours are more complex, use the Polygon Form.
Figure XX (a) HSRC/MEx image footprints. (b) individual spectra (iFoV) from VIRTIS/Rosetta cubes, in SkyPlot with lon/lat. (c) same in PlanePlot with Cartesian coord.
• If the table contains footprints provided through an STC-S Polygon string (in particular s_region in EPN-TAP tables):
- define a new column (e.g. "shape") with expression "parseDoubles(s_region)"
- use Lon=shape, Lat=shape as positions in Skyplot
- add a new Form of type Polygon, and fill in Other Points field as "shape"
This applies either to image contours, to individual FoV for spectra, and to other situations:
Figure: Mars craters contours.
Plotting on shape models in TOPCAT
If the table contains the Cartesian coordinates of individual measurements, TOPCAT can easily display this in 3D:
(the shape model is then actually included in the data - but only where measurements exist)
• Open a 3D Cartesian plot window. X/Y/Z coordinates are preselected if the names are explicit enough. This also works in 2D plots, with all Z values projected in the X/Y plane.
• The usual plot options are available to superpose an Aux parameter.
Figure: VIRTIS/Rosetta measurements on 67P, in Cartesian coordinates
Looking for spatial overlaps between datasets
This uses TAP functions on the s_region parameter. Such requests can be issued from TOPCAT using a subset table select in the VESPA portal.
A tutorial is available here:
This includes mapping examples in TOPCAT and Aladin.
Mars HiPS generation in Aladin
HiPS (Hierarchical progressive survey) are associations of healpix maps at different scales. They provide efficient multiresolution scaling on the fly in Aladin.
1) From a single file (updated May 2019, v10.128)
The use is mostly to get a 3D spherical model from a map, where contours and objects can be overplotted. See Io use case for more details: Aladin & planetary surfaces
In Aladin, load file Mars_MGS_colorhillshade_mola_1024.jpg (MOLA integrated relief map)
• Go to Image > astrometric calibration:
Coord = 0.,0.
pos x/y = 512, 256 (lon = 0°, lat = 0° at image center)
size = 21.1' (= 360. / 1024)
RA symmetry = True => this is mandatory to get the image in the correct orientation.
• Convert to HiPS :
Tool > Generate HiPS based on… > current image
• The hpx image is saved (somewhere) on disk automatically (you may have to search for it)
• Select the HiPS and set Properties => .longitudes = ascending ; will plot correctly with E longitudes eastward (the button exists but this option is not working in v10.127)
If color scale goes crazy, fix it by clicking the Pixel icon, then Reverse button.
• s_region footprints sent from VESPA (with E longitudes) will plot at the right place.
• s_region footprints retrieved directly by Aladin from an EPN-TAP service (with E longitudes) will also plot at the right place.
2) From a series of local files in Aladin
This method will provide adaptive resolution when using files of various scales
TBC, did not work on my machine early 2017.
MOC use case in Aladin
(updated May 2019)
MOC (Multi-Order Coverages) are footprints defined in terms of a healpix cells list mixing various resolutions. They provide immediate location information e.g. about intersections, and can accommodate footprints of arbitrary complexity (hollowed, non-connex, etc…). There is no explicit contour associated, so the usage is different from the s_region parameter.
- Should also retain info about original file coverages, for callback purpose.
- Load previous Mars MOLA HiPS in Aladin
- Select two large craters in Mars Craters service from VESPA client
- send as s_region from VESPA client (to Aladin) — (s_region was then provided in this service with W longitudes, so this projected correctly)
- select them in Aladin with pointer
- Generate common MOC for these 2 craters (Coverage > Generate a MOC based on… > selected drawing objects)
- You can save it from File > Export Planes (save preferably as fits for reuse)
This is straightforward and very quick.
This is not a direct alternative to s_region however: the result is a list of healpix cells which is much larger than a contour, and can't be included in a table. But we can either have precomputed MOC stored somewhere (for complete datasets? Especially with time direction added), or compute them on the fly when required.
• Next step: test intersections with something else - see example in the next pages