[daip] From Ashok Singal - A question about contour plots in AIPS

[Eric Greisen]

On 09/27/2016 01:34 AM, A K Singal wrote:
> Hi Sanjay and Vivek,
> It is ages since I saw either of you. Last I had seen your pictures which
> Kamlesh had sent me when she was vising Socorro a couple of years back.
> Hope you are fine with your extended families (two+?).
>
> I recently noticed a curious problem with AIPS contour plotting (it might
> be just due to my ignorance but still!). Whenever there is a depression in
> intensity surrounded on all sides by brighter regions, the contour plots
> do not indicate the decrease in intensity in the depression region, and in
> fact the contours show as if the intensity is higher in the depression.
> For example a crator surrounded by rim of the volcano when contours
> plotted by AIPS seem to showing the crator to be higher than the rim I am
> attaching a figure of IC711 (a head-tail radio galaxy, which we observed
> with the GMRT) where `A' looks to be a bright point, while in the  grey
> scale map it is correctly shown to be a dark region. I noticed similar
> problems in the published maps in literature too, e.g. 3C129 has almost
> the same problem (also included in my fig being sent here). Contour maps
> which have more quantitative information could present a wrong picture in
> such cases.
>
> May be it is only my ignorance, in case there is a way to handle such
> cases in AIPS, can you inform me? I just checked that geology people use
> contours with ticks marked inside, little 'teeth' called HACHURES! For
> hachured contours, a point inside a contour is lower than the contour and
> a point outside a contour is higher than the contour. Just the opposite of
> ordinary, unhachured contours!
> I do not know whether a similar convention is used in AIPS, at least I did
> not find and it does not happen as by default.
>
> Best regards,
> Ashok
>
AIPS contour plotting simple plots the contours where they lie (but uses 
dashed contours for negative levels)  The hachure algorithm you describe 
is rather complicated and is not done in AIPS.  This is why the 
grey-scale options are important.  You could of course use colored 
contours and then it should be clear where the minima occur.

ERic Greisen