[GIS] Qgis – need to draw exact lines (mtrs) with fixed angle

lengthlineqgis

I use GIS 1.8.

I have the need to be able to draw exact lines and shapes on a bing map.
(This is to project a yet to be build greenhous project on a map to show what i would look like).

I have found the tool of Olivier Dalang (improved polygon capture) to be very usefull, since this tool enables me to draw lines while specifying the exact length and angle of the lines, and by doing so i build all walls of the greenhouse on the map).

Here is my problem.
The plugin enables me to enter the length in 'units', and drawing a line of 1000 units makes me come up with a line of about 622 meters.

I cannot find the exact way to change or find out how to draw in meters.

Any help highly appreciated!

(If there are other tools for drawing 90 degrees angled lines to a map I welcome these too!)

Best Answer

If you use bing map, you may use Openlayers plugin. The units of that are not real metres. They fit only at the equator, and get distorted the more to the poles you come.

Try the UTM projection designed for your part of the world to draw the building with cad tools.

If you don't have a suitable map in UTM projection, make a screen copy of the bing map with Save as picture, load it as raster file with EPSG:3857 as layer CRS, change the project CRS to your UTM, create the vector layer in the same CRS, and draw your building.

Then you can switch the project CRS back to EPSG:3857, delete the raster picture layer and activate the bing background again.

1. Get the slope of the lines

I am not sure of the lines between which is the angle.

If between the line and the x axis: $slope = tan\theta$

If between the origin and a given point: $slope=tan(arctan\frac{y}{x} - \theta)$

2. Get the parametric equation of the lines

This can be achieved with the traditional line equation $y = ax +b$ or with the point-slope equation $y - y_1 = m(x - x_1)$ . In both case, this (the origin b or the point) will be obtained based on the spacing between the lines.

3. Create the segment

With the equation of each line, we can create a segment contained withing the rectangle. For this step, I would use the functions in the shapely library (union, intersection,...)

4. Export that in whatever format

[GIS] How to draw a simple bus network that works with pgRouting

What you need for pgRouting - at minimum - is a table with edges:

edge_id, from_node, to_node, cost
1, 1, 2, 12.0
2, 2, 3, 57.0

Note that geometries of the edges are not necessary for most algorithms. Only the logical topology has to be created.

If you want to have edge geometries: start from the bus/train stop layer and create one edge for every connection (with snapping to the stops layer enabled). You can chose between assigning from_node and to_node ids manually or using one of the built-in pgRouting functions. For the automated process to work, you have to do a good job while digitizing the geometries.

Depending on how many bus/train lines you are planning to add, it might be advisable to try to automate as much of this process as possible. Is the "sort" attribute included in your bus stop data or did you have to add that manually?

Previous answer:

Since the ends of the two lines you show in the screenshot are very far apart, you would have to set the tolerance in pgr_createTopology really high (http://docs.pgrouting.org/dev/src/common/doc/functions/create_topology.html).

Instead you should pay more attention to digitizing the lines correctly by enabling snapping.

Best Answer

Related Solutions

[GIS] draw lines with an angle inside a rectangle in python

1. Get the slope of the lines

2. Get the parametric equation of the lines

3. Create the segment

4. Export that in whatever format

[GIS] How to draw a simple bus network that works with pgRouting

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