[Qgis-user] Problem with Rasterize (Vector to raster) tool

Fri Apr 11 03:06:58 PDT 2014

Thanks Saber, Micha and Carlos for your responses,

I tried the v.to.rast.attribute route (and also v.rast.value), but couldn’t get those tool to work with my data set (and I’m still exploring this issue and trying to work out what’s going on as they look like useful tools to be able to use).

However, returning to the original Rasterize tool, I examined the raster data layer which was created in more detail and found that while the histogram was saying that the maximum value was 0.995968, when I interrogated the individual grid cells with the Interrogate Features tool, the values were indeed one where they should be. Thus, my problem was not that the Rasterize tool was not working, but rather that it wasn’t being displayed correctly in QGIS. 

To deal with this, I went into the Layer Properties and under the Style tab, I changed the maximum value from 0.995968 to 1. This made the legend in the Table Of Contents window look a little odd, but the layer displayed correctly, has the right value in it, and does the correct calculations when using the raster calculator tool. 

I’m not too sure why the layer is not being displayed correctly in the Table Of Contents window in the first place. Similarly, if I remove the raster data layer from the my project and add it back in again, it reverts to saying the maximum value is 0.995968 rather than 1 in the legend in the Table Of Contents window and in the histogram for the data layer. Thus, each time I use it in a new project, I have to manually change the maximum number to 1. 

I don’t know if this is a bug as such, or if this happens with other data sets (it happened with both of mine – one point and one line data layer), but it is certainly a potential pitfall for the novice user when it occurs.

All the best,

Colin

==================================================================================
GIS IN ECOLOGY - Providing Training, Advice And Consultancy On The Use Of GIS In Ecology

Web: www.GISinEcology.com Email: info at GISinEcology.com

Need to ask a question about using GIS? Try the GIS In Ecology Forum: www.GISinEcology.com/GIS_in_Ecology_forum.htm

Books From GIS In Ecology Staff:

An Introduction To Using GIS In Marine Biology; RRP: £44.99
An Introduction To Using GIS In Marine Biology: Supplementary Workbook One - Creating Maps Of Species Distribution; RRP: £19:99 
An Introduction To Using GIS In Marine Biology: Supplementary Workbook Two - Working With Raster Data Layers; RRP: £19.99 

If you wish to purchase these books, visit: http://www.gisinecology.com/Book_Shop.htm

To help the environment, please do not print out this email unless it is unavoidable.
==================================================================================

From: Carlos Cerdán 
Sent: Thursday, April 10, 2014 9:23 PM
To: qgis-user at lists.osgeo.org 
Subject: Re: [Qgis-user] Problem with Rasterize (Vector to raster) tool

Perhaps raster calculator can also help you

Expression:

(layer>0 and layer <= 1)

Gives us 1 when sentence is true and 0 for false, for each cell in "layer".

Good luck

Carlos

2014-04-10 9:29 GMT-05:00 Micha Silver <micha at arava.co.il>:

  Using GRASS there are two modules you can consider. First :
  v.to.rast in=... out=... type=point use=val value=1
  will convert points into raster cells (at the current region settings), where any cell that contains at least one point will be given value 1.

  Next:
  r.in.xyz in=... out=... method=sum 
  will create a raster (at the current region settings) where each cell will have a value = the count of points in that cell.

  HTH,
  Micha 

  On 04/10/2014 04:20 PM, Colin D. MacLeod wrote:

    Hi All,

    I’m a fairly experienced GIS user, but I’m new to QGIS. I’m using version 2.2, and I’m trying to convert a point data layer into a raster data layer using the Rasterize tool. The point data layer has an integer field with a value of 1 for all records, and I want to make a raster data layer which has a value of one for each cell which has at least one point in it (some cells have more than one point in them). When I use the Rasterize tool, it creates the raster data layer fine, but all cells with have points in them appear to have a value of 0.995968. I’ve tried editing the gdal code (even making the resulting raster data layer an integer layer), but I still cannot get it to produce a raster with a value of 1 rather than just a number very close to one. 

    Does anyone have any suggestions of where I might be going wrong and/or how to solve this issue?

    On a related note, is it possible to use the Rasterize tool to work out the average or total values for all the points which fall in each grid cell? If so, how (I can’t find anything in the gdal instructions themselves which would suggest this is possible, but I might be missing something). If this isn’t possible to do with the Rasterize tool, are there other tools in QGIS which can do this?

    Thanks for any help anyone can give on this.

    All the best,

    Colin

    ==================================================================================
    GIS IN ECOLOGY - Providing Training, Advice And Consultancy On The Use Of GIS In Ecology

    Web: www.GISinEcology.com Email: info at GISinEcology.com

    Need to ask a question about using GIS? Try the GIS In Ecology Forum: www.GISinEcology.com/GIS_in_Ecology_forum.htm

    Books From GIS In Ecology Staff:

    An Introduction To Using GIS In Marine Biology; RRP: £44.99
    An Introduction To Using GIS In Marine Biology: Supplementary Workbook One - Creating Maps Of Species Distribution; RRP: £19:99 
    An Introduction To Using GIS In Marine Biology: Supplementary Workbook Two - Working With Raster Data Layers; RRP: £19.99 

    If you wish to purchase these books, visit: http://www.gisinecology.com/Book_Shop.htm

    To help the environment, please do not print out this email unless it is unavoidable.
    ==================================================================================

    This mail was received via Mail-SeCure System.

_______________________________________________
Qgis-user mailing list
Qgis-user at lists.osgeo.org
http://lists.osgeo.org/mailman/listinfo/qgis-user
This mail was received via Mail-SeCure System.

  _______________________________________________
  Qgis-user mailing list
  Qgis-user at lists.osgeo.org
  http://lists.osgeo.org/mailman/listinfo/qgis-user

--------------------------------------------------------------------------------
_______________________________________________
Qgis-user mailing list
Qgis-user at lists.osgeo.org
http://lists.osgeo.org/mailman/listinfo/qgis-user
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.osgeo.org/pipermail/qgis-user/attachments/20140411/e01a8d54/attachment.html>