[GRASS-user] Update categories/attribute table after v.clean with option break

GRASS GRASS user
1

Hi,

I have a vector line map (river network) which I want to clean and break the lines at intersections. Therefore, I use the tool v.clean with the option break. This of course increases the number of vector lines as several lines got broken into two or more separate lines. Now there is a attribute table linked to the original map. I want to update this attribute table by inserting a new column that contains a new unique ID (cat) for each line but still keeps the old cat in another column (E.g. old_cat). Thus the attribute table should also increase in number of rows (=number of lines).

Now, most likely the tool to use now is v.category to get a column of new unique categories (in another table/layer?) and the use probably a join statement to get back the information from the original table (with doubled entries for those lines that got broken during v.clean).

What I tried so far:

#########################

Cleaning vector and breaking lines

v.clean -c --overwrite input=river_network_modified output=river_network_modified_clean tool=break,snap,rmline threshold=0,50,0

Check cats

v.category input=river_network_modified_clean option=report layer=-1

type count min max

line 16592 1 16448

Update cats

v.category -t --overwrite input=river_network_modified_clean type=line output=river_network_modified_clean2 option=del
v.category -t --overwrite input=river_network_modified_clean2 type=line output=river_network_modified_clean3 option=add

Check cats

v.category input=river_network_modified_clean2 option=report layer=-1

Layer: 1

type count min max

line 16591 2 16448

Check cats

v.category input=river_network_modified_clean3 option=report layer=-1

Layer: 1

type count min max

line 16592 1 16448

########################

However, I expected the max values of the latest (updated) vector to be 16592 to have really unique values for all 16592 lines (from 1 to 16592)? But the output shows the same information as the initial input map? Probably I am still mixing up things with categories/layers… and maybe someone has a quick hint how to get an updated attribute table for a map that has been processed using v.clean and where lines were broken.

/johannes

2

On Wed, Sep 7, 2016 at 10:06 AM, Johannes Radinger
<johannesradinger@gmail.com> wrote:

Hi,

I have a vector line map (river network) which I want to clean and break the
lines at intersections. Therefore, I use the tool v.clean with the option
break. This of course increases the number of vector lines as several lines
got broken into two or more separate lines. Now there is a attribute table
linked to the original map. I want to update this attribute table by
inserting a new column that contains a new unique ID (cat) for each line but
still keeps the old cat in another column (E.g. old_cat). Thus the attribute
table should also increase in number of rows (=number of lines).

Now, most likely the tool to use now is v.category to get a column of new
unique categories (in another table/layer?) and the use probably a join
statement to get back the information from the original table (with doubled
entries for those lines that got broken during v.clean).

What I tried so far:

#########################
# Cleaning vector and breaking lines
v.clean -c --overwrite input=river_network_modified
output=river_network_modified_clean tool=break,snap,rmline threshold=0,50,0

# Check cats
v.category input=river_network_modified_clean option=report layer=-1
# type count min max
# line 16592 1 16448

# Update cats
v.category -t --overwrite input=river_network_modified_clean type=line
output=river_network_modified_clean2 option=del

v.category option=del deletes the selected category value for the
selected layer and selected geometry types. By default, categories
with value 1 in layer 1 are deleted for all points, lines, centroids,
and faces. This is probably not what you want.

Instead, you could try to add new categories to a new layer, e.g. with

v.category option=add layer=2 cat=1 step=1 type=line

Now add a new table to layer 2
v.db.addtable layer=2 columns="cat_lyr1 integer"

Load category values (or any other attributes) from layer 1 to layer 2 with
v.to.db layer=2 column=cat_lyr1 query_layer=1 query_column=cat

Now you have unique new categories in layer 2 and the old categories
still existing in layer 1 are also in the field cat_lyr1 of layer 2.

HTH,

Markus M

v.category -t --overwrite input=river_network_modified_clean2 type=line
output=river_network_modified_clean3 option=add

# Check cats
v.category input=river_network_modified_clean2 option=report layer=-1
# Layer: 1
# type count min max
# line 16591 2 16448

# Check cats
v.category input=river_network_modified_clean3 option=report layer=-1
# Layer: 1
# type count min max
# line 16592 1 16448
########################

However, I expected the max values of the latest (updated) vector to be
16592 to have really unique values for all 16592 lines (from 1 to 16592)?
But the output shows the same information as the initial input map? Probably
I am still mixing up things with categories/layers... and maybe someone has
a quick hint how to get an updated attribute table for a map that has been
processed using v.clean and where lines were broken.

/johannes

_______________________________________________
grass-user mailing list
grass-user@lists.osgeo.org
http://lists.osgeo.org/mailman/listinfo/grass-user

3

Hi,

I have attempted to make it a FAQ page:

https://grasswiki.osgeo.org/wiki/Vector_break_line_intersections

Please feel free to edit and improve this Wiki page.

markusN