Hi list,
I have a whole bunch of data from an RTK GPS. File exists as a MapInfo file. Imports into grass fine with area and centroids.
My problem is however, i want to create another vector file that contains the mask or the external (an internal island) boundary.
I have been able to create the external boundary fine through the following.
v.category (delete categories)
v.category (add category as 1 for all)
v.dissolve (combine all boundaries with common category)
This is great and works fine for the external boundary. However there is meant to be an island in there. Somewhere along the way (i think after the second call to v.category) it fills that island in.
Has anyone got any hints or ideas?
Essentially i am trying to create this vector to generate a mask for further interpolation of the point heights. I am assuming that i need this island removed (as there is no point data for it) otherwise it is going to effect the interpolation.
Thanks for your time.
Ed
Hi list,
My apologies. The island is actually and obstacle so I still need to include it.
However, is there a better way of achieving the external boundary using the centroids or points??
Thanks,
Ed
2008/5/29 Edward Tomlinson <e.j.tommo@gmail.com>:
Hi list,
I have a whole bunch of data from an RTK GPS. File exists as a MapInfo file. Imports into grass fine with area and centroids.
My problem is however, i want to create another vector file that contains the mask or the external (an internal island) boundary.
I have been able to create the external boundary fine through the following.
v.category (delete categories)
v.category (add category as 1 for all)
v.dissolve (combine all boundaries with common category)
This is great and works fine for the external boundary. However there is meant to be an island in there. Somewhere along the way (i think after the second call to v.category) it fills that island in.
Has anyone got any hints or ideas?
Essentially i am trying to create this vector to generate a mask for further interpolation of the point heights. I am assuming that i need this island removed (as there is no point data for it) otherwise it is going to effect the interpolation.
Thanks for your time.
Ed
On Thu, 2008-05-29 at 09:41 +1000, Edward Tomlinson wrote:
Hi list,
[...]
My problem is however, i want to create another vector file that
contains the mask or the external (an internal island) boundary.
I have been able to create the external boundary fine through the
following.
v.category (delete categories)
v.category (add category as 1 for all)
v.dissolve (combine all boundaries with common category)
This is great and works fine for the external boundary. However there
is meant to be an island in there. Somewhere along the way (i think
after the second call to v.category) it fills that island in.
Has anyone got any hints or ideas?
If it is about one "area" only what about removing it's centroid before
dissolving (I guess) ?
[...]
Nikos
Edward Tomlinson:
I have a whole bunch of data from an RTK GPS. File exists
as a MapInfo file.
Imports into grass fine with area and centroids.
My problem is however, i want to create another vector file
that contains the mask or the external (an internal island) boundary.
I have been able to create the external boundary fine
through the following.
v.category (delete categories)
v.category (add category as 1 for all)
v.dissolve (combine all boundaries with common category)
This is great and works fine for the external boundary.
However there is meant to be an island in there. Somewhere
along the way (i think after the second call to v.category)
it fills that island in.
right.
Has anyone got any hints or ideas?
Essentially i am trying to create this vector to generate a
mask for further interpolation of the point heights. I am
assuming that i need this island removed (as there is no
point data for it) otherwise it is going to effect
the interpolation.
Use v.overlay for the new and original map, operator=not, etc.
or other combination of operator=and etc until you get what you want.
another trick is "v.extract list=1-99999999" to only extract features with a cat number. (then v.overlay)
v.hull is another module to look at for this instead of v.dissolve.
Hamish
Hi Hamish,
Thank you for the suggestions. v.hull is something that i had passed over before, it wont work on this occasion as the dam is boomerang shaped, but i will be sure to use it when i can. Takes a fair deal of processing out.
For those that are interested the final result was as follows:
v.category option=add cat=1 step=0 type=area (this filled in my islands - as they weren’t islands, just obsticales)
v.extract type=area new=1 list=1-99999
v.dissolve
Thanks,
Ed
2008/5/29 Hamish <hamish_b@yahoo.com>:
Edward Tomlinson:
I have a whole bunch of data from an RTK GPS. File exists
as a MapInfo file.
Imports into grass fine with area and centroids.
My problem is however, i want to create another vector file
that contains the mask or the external (an internal island) boundary.
I have been able to create the external boundary fine
through the following.
v.category (delete categories)
v.category (add category as 1 for all)
v.dissolve (combine all boundaries with common category)
This is great and works fine for the external boundary.
However there is meant to be an island in there. Somewhere
along the way (i think after the second call to v.category)
it fills that island in.
right.
Has anyone got any hints or ideas?
Essentially i am trying to create this vector to generate a
mask for further interpolation of the point heights. I am
assuming that i need this island removed (as there is no
point data for it) otherwise it is going to effect
the interpolation.
Use v.overlay for the new and original map, operator=not, etc.
or other combination of operator=and etc until you get what you want.
another trick is “v.extract list=1-99999999” to only extract features with a cat number. (then v.overlay)
v.hull is another module to look at for this instead of v.dissolve.
Hamish