Grid registration - Revision history

⚠️HamishBowman: /* Example */

2010-01-04T13:00:18Z

Example

← Older revision		Revision as of 13:00, 4 January 2010
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	<source lang="bash">		<source lang="bash">
	# scan ~~the~~ file:		# scan a text file containing gridded x,y,z data:
	r.in.xyz -s -g in=tooth_scan.txt fs=, out=dummy		r.in.xyz -s -g in=tooth_scan.txt fs=, out=dummy
	n=10.550000 s=3.850000 e=5.800000 w=1.050000 b=2.900000 t=4.875000		n=10.550000 s=3.850000 e=5.800000 w=1.050000 b=2.900000 t=4.875000
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	# will automatically align back to the grid. Save for backup:		# will automatically align back to the grid. Save for backup:
	g.region save=tooth		g.region save=tooth
			</source>

			To check if the region alignment is set correctly, create a ''n''
			map and enusre that you are in fact importing one point per cell:
			<source lang="bash">
			r.in.xyz in=tooth_scan.txt fs=, out=tooth.n method=n --verbose
			r.univar tooth.n

	r.in.xyz in=tooth_scan.txt fs=, out=tooth ~~--verbose~~		# if ''n'' is 1 thoughout, min,max,median, and mean maps should all be identical.
			r.in.xyz in=tooth_scan.txt fs=, out=tooth method=mean

	# the old looks-good-with-everything standby color table:		# the old looks-good-with-everything standby color table:

⚠️HamishBowman: /* Grid vs. cell-center registration */

2010-01-03T10:08:43Z

Grid vs. cell-center registration

← Older revision		Revision as of 10:08, 3 January 2010
Line 5:		Line 5:

	If you have gridded data -and do not need to interpolate- then		If you have gridded data -and do not need to interpolate- then
	{{cmd\|r.in.xyz}} is the perfect tool, you just have to remember that '''GRASS		{{cmd\|r.in.xyz}} is the perfect tool, you just have to remember that '''GRASS uses cell-center registration not grid line registration'''.
	uses cell-center registration not grid line registration'''.

	* if there are holes to interpolate you might as well just import as points directly to skip a {{cmd\|r.to.vect}} step (assuming {{cmd\|v.surf.rst}})		* if there are holes to interpolate you might as well just import as points directly to skip a {{cmd\|r.to.vect}} step (assuming {{cmd\|v.surf.rst}})

⚠️HamishBowman: /* Grid vs. cell-center registration */

2010-01-03T09:45:29Z

Grid vs. cell-center registration

← Older revision		Revision as of 09:45, 3 January 2010
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	0,0 has little meaning in this coord system..		0,0 has little meaning in this coord system..

	* For something like sparse ~~lidar~~ data it doesn't really matter as there is no grid aliasing effect to worry about and r.in.xyz includes points falling exactly on the outer boundary of the region*		* For something like sparse [[LIDAR]] data it doesn't really matter as there is no grid aliasing effect to worry about and r.in.xyz includes points falling exactly on the outer boundary of the region*
	: [*] except for the bottom boundary; I haven't gotten to that yet (see trac)		: [*] except for the bottom boundary; I haven't gotten to that yet (see trac)

⚠️HamishBowman: /* Example */

2010-01-03T09:42:29Z

Example

← Older revision		Revision as of 09:42, 3 January 2010
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	# set the region based on that:		# set the region based on that:
	g.region n=10.550000 s=3.850000 e=5.800000 w=1.050000 res=$RES -p		g.region n=10.550000 s=3.850000 e=5.800000 w=1.050000 res=$RES -p

	north: 10.55		north: 10.55
	south: 3.85		south: 3.85
Line 81:		Line 82:

	g.region n=n+$RES2 s=s-$RES2 w=w-$RES2 e=e+$RES2 -p		g.region n=n+$RES2 s=s-$RES2 w=w-$RES2 e=e+$RES2 -p

	north: 10.575		north: 10.575
	south: 3.825		south: 3.825
Line 94:		Line 96:
	# will automatically align back to the grid. Save for backup:		# will automatically align back to the grid. Save for backup:
	g.region save=tooth		g.region save=tooth


	r.in.xyz in=tooth_scan.txt fs=, out=tooth --verbose		r.in.xyz in=tooth_scan.txt fs=, out=tooth --verbose

⚠️HamishBowman: /* Grid vs. cell-center registration */

2010-01-03T09:41:35Z

Grid vs. cell-center registration

← Older revision		Revision as of 09:41, 3 January 2010
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	n=800 s=550 w=300 e=600 res=50		n=800 s=550 w=300 e=600 res=50

	and the raster /appears/ shifted by 1/2 a cell when you plot it.		and the raster ''appears'' shifted by 1/2 a cell when you plot it.


Line 52:		Line 52:
	: [*] except for the bottom boundary; I haven't gotten to that yet (see trac)		: [*] except for the bottom boundary; I haven't gotten to that yet (see trac)


			=== Example ===

	<source lang="bash">		<source lang="bash">

⚠️HamishBowman: init

2010-01-03T09:40:40Z

init

New page

== Grid vs. cell-center registration ==

''a subtle beast....''

If you have gridded data -and do not need to interpolate- then
{{cmd|r.in.xyz}} is the perfect tool, you just have to remember that '''GRASS
uses cell-center registration not grid line registration'''.

* if there are holes to interpolate you might as well just import as points directly to skip a {{cmd|r.to.vect}} step (assuming {{cmd|v.surf.rst}})

So we want the height (depth?) points to be in the center of each
cell in our region, but r.in.xyz's scan gives us the outer bounding
box (ie outer grid lines).

Thus for gridded data you have to grow the scanned max extent by
1/2 a cell outwards if you want to use the max extent numbers.

A problem then is that '{{cmd|g.region}} -a' and {{cmd|d.zoom}} will align the
new region to the resolution & undoing your 1/2 cell. I'm not sure
if the GUI zooms do that or not. After it gets damaged by d.zoom
it renders as if the vector points are on the grid points, but
really they aren't and halving the resolution or growing the
region out by 1/2 a cell again gets them back to normal.

consider:
n=1025 s=275 w=125 e=775 res=50

d.zoom (etc) will see the res=50 and want to grow it outwards to the
next even multiple of res, so after d.zoom the region might be like:

n=800 s=550 w=300 e=600 res=50

and the raster /appears/ shifted by 1/2 a cell when you plot it.

In most cases this would be what you want to happen, but not when you have a grid-registration mismatch to deal with.

A q&d solution to all this is to halve the resolution again
after import. BUT- do set the region back to the real value before
starting NVIZ or else the results look really blocky. (or setting the fine
resolution to "2" will do pretty much the same thing)

Another idea is to use {{cmd|r.region}} to shift the data by half a cell up
and to the right. in this case the coordinate system is relative so
we can set our "false eastings" and "y_0"s to anything we like..
0,0 has little meaning in this coord system..

* For something like sparse lidar data it doesn't really matter as there is no grid aliasing effect to worry about and r.in.xyz includes points falling exactly on the outer boundary of the region*
: [*] except for the bottom boundary; I haven't gotten to that yet (see trac)

<source lang="bash">
# scan the file:
r.in.xyz -s -g in=tooth_scan.txt fs=, out=dummy
n=10.550000 s=3.850000 e=5.800000 w=1.050000 b=2.900000 t=4.875000

# apply what we know:
RES=0.05

# calc what half of that is:
RES2=`echo "$RES" | awk '{print $1 / 2}'`

# set the region based on that:
g.region n=10.550000 s=3.850000 e=5.800000 w=1.050000 res=$RES -p
north: 10.55
south: 3.85
west: 1.05
east: 5.8
nsres: 0.05
ewres: 0.05
rows: 134
cols: 95
cells: 12730

g.region n=n+$RES2 s=s-$RES2 w=w-$RES2 e=e+$RES2 -p
north: 10.575
south: 3.825
west: 1.025
east: 5.825
nsres: 0.05
ewres: 0.05
rows: 135
cols: 96
cells: 12960

# Now we have to be very careful, as 'g.region -a' and d.zoom
# will automatically align back to the grid. Save for backup:
g.region save=tooth

r.in.xyz in=tooth_scan.txt fs=, out=tooth --verbose

# the old looks-good-with-everything standby color table:
r.colors tooth color=bcyr
# my new favorite color table (in svn addons)
r.colors tooth col=haxby -n
# something more natural:
r.colors tooth color=sepia

# 'r.colors -e' can help to bring out the pic a bit

d.grid 1 # defaults are just right here

####

v.in.ascii in=tooth_scan.txt fs=, out=Tooth x=1 y=2 z=3

d.vect Tooth -z size=2 zcolor=bcyr

g.region res=$RES2 -p

d.zoom ...

####

# first thing after setting the z-exag to 1.0 go into the lighting
# controls and set lights to follow viewpoint or else the shadows getcha
g.region tooth
nviz tooth
</source>

[[category: FAQ]]
[[category: raster]]