GAMMA Questions

Hello,
I quote from the documentation for gc_map2:

Layover-and-shadow maps can be generated in both MAP and slant range geometry
using|ls_map|and|ls_map_rdc|options. The|r_ovr|option determines the ground
range sampling used for the layover-and-shadow map calculation (default value:
2). A larger oversampling factor may improve layover-and-shadow map quality. Two
scales are available: either default values or visualization-optimized values
can be written out using the|ls_scaling|option. The default values use 0 for
"not tested", 1 for "tested", 4 for "layover", 16 for "shadow", and 20 for
"layover-in-shadow". The values can are logically mixed together (using the "or"
operator): a pixel that was "tested" and detected as "layover" will have a value
equal to|1 or 4 = 5|. These default values are compatible with other programs,
such aspixel_area. The visualization-optimized values use 0 for "not tested",
127 for "tested", 255 for "layover", 31 for "shadow", and 63 for
"layover-in-shadow". That option creates an image scaled relatively close to
simulated SAR data but should only be used for visualization purposes
(using/disbyte/or/rasbyte/programs).

The layover shadow map is quite accurate, it is calculated in the slant-range
geometry . Use the the*/ls_scaling/* option to produce a visualization better
suited for screen display!
The default values for the different flags e.g.( layover, layover in shadow,
shadow...) are values not well suited for screen display, but are compatible
with other programs in the Gamma software.

Calculation of the accurate layover shadow map produced by gc_map2 is quite
costly computationally and not necessary for geocoded the data.
Nominally I do not calculate it in order to save time (set ls_map and ls_map_rdc
flags to - ).

Best regards,
Charles

Dear Sir, Dr. Charles,
Thank you for a detailed reply. Yes, ls-map won't be necessary, but I need to at least visualize this in order to analyze and validate the final results of the hilly area.
When I compare this with the previous gc_map using ls-mode as '2', I can see four values i.e. 1,5,7 and 25 which are the true values of the lookup table without ls. Can we interpret these values '5' and '7' as layover? What I need is to visualize a map to show and divide the portions of layover, shadow etc.

Thanks
Naeem

Hello,
In gc_map ls_mode 2 has nothing to do with visualization of the layover-shadow.
ls_mode 2 in gc_map permits multiple slant range points to map to the same point
in the map projection. Layover areas will appear very bright.
The other modes (1: (linear interpolation), or 0: (blank) cause their own
artifacts or holes in the image.

The main reason we wrote gc_map2 is to generate accurate layover and shadow maps
and calculates the lookup table using the equivalent of LS_mode 2 in gc_map. I
suggest that you not use gc_map to generate layover shadow maps since their
accuracy is modest.

Just use gc_map2 with ls_scaling so that you can visualize the data better. 
(ls_scaling 1). The interpretation of values that are produced when (ls_scaling:
0) are as below. These values set different binary bits in the layover shadow mask:

#define NOT_TESTED 0
#define TESTED 1
#define LAYOVER 4
#define SHADOW 16
#define SHADOW_IN_LAYOVER 20

tested/not tested : bit 0
layover/no layover:  bit 2
shadow/no shadow:    bit 3

So a bit-wise logical or of the masks gives the condition, for example:

shadow + layover =  00001100b = 20

Best regards,
Charels

Hello Dr. Charles,

Many Thanks for this. Please see the attached jpgs with ls_scaling as default value and with the value 1. Does the brightest part means the layover over the whole image?

Regards
Naeem