Tag: vector

Geospatial Power Tools Reviews [Book]

Thinking of buying my latest book?  We’ve finally got a few reviews on Amazon that might help you decide.  See my other post for more about the book.  Buy the PDF on Locate Press.com.

Reader Reviews

Geospatial Power Tools book cover
From Amazon.com

5.0 out of 5 stars This book makes a great reference manual for using GDAL/OGR suite of command line …,
January 24, 2015 By Leo Hsu
“The GDAL Toolkit is chuckful of ETL commandline tools for working with 100s of spatial (and not so spatial data sources). Sadly the GDAL website only provides the basic API command switches with very few examples to get a user going with. I was really excited when this book was announced and purchased as soon as it came out. This book makes a great reference manual for using GDAL/OGR suite of command line utilities.
Several chapters are devoted to each commandline tool, explaining what its for, the switches it has, and several examples of how to use each one. You’ll learn how to work with both vector/(basic data no vector) data sources and how to convert from one vector format to another. You’ll also learn how to work with raster data and how to transform from one raster data source to another as well as various operations you can perform on these.”

 

Continue reading Geospatial Power Tools Reviews [Book]

Create a Union VRT from a folder of Vector files

The following is an excerpt from the book: Geospatial Power Tools – Open Source GDAL/OGR Command Line Tools by me, Tyler Mitchell.  The book is a comprehensive manual as well as a guide to typical data processing workflows, such as the following short sample…

The real power of VRT files comes into play when you want create virtual representations of features as well.  In this case, you can virtually tile together many individual layers as one.  At the present time you cannot do this with a single command line but it only takes adding two simple lines to the VRT XML file to make it start working.

Here we want to create a virtual vector layer from all the files containing lines in the ne/10m_cultural folder.

First, to keep it simple, create a folder and copy in only the files we are interested in:

mkdir ne/all_lines 
cp ne/10m_cultural/*lines* ne/all_lines

Then we can create our VRT file using ogr2vrt as shown in the previous example:

python ogr2vrt.py -relative ne/all_lines all_lines.vrt

If added to QGIS at this point, it will merely present a list of four layers to select to load. This is not what we want.

So next we edit the resulting all_lines.vrt file and add a line that tells GDAL/OGR that the contents are to be presented as a unioned layer with a given name (i.e. “UnionedLines”).

The added line is the second one below, along with the closing line second from the end:

<OGRVRTDataSource>
 <OGRVRTUnionLayer name="UnionedLines">
  <OGRVRTLayer name="ne_10m_admin_0_boundary_lines_disputed_areas">
   <SrcDataSource relativeToVRT="1" shared="1">
   ...
   <Field name="note" type="String" src="note" width="200"/>
  </OGRVRTLayer>
 </OGRVRTUnionLayer>
</OGRVRTDataSource>

Now loading it into QGIS automatically loads it as a single layer but, behind the scenes, it is a virtual representation of all four source layers.

On the map in Figure 5.8 the unionedLines layer is drawn on top using red lines, whereas all the source files (that I manually loaded) are shown with a light shading. This shows that the new virtual layer covers all the source layer features.

Unioned OGR VRT layers - source layers beneath final resulting merged layer
Unioned OGR VRT layers – source layers beneath final resulting merged layer

 

Geospatial Power Tools is 350 pages long – 100 of those pages cover these kinds of workflow topic examples.  Each copy includes a complete (edited!) set of the GDAL/OGR command line documentation as well as the following topics/examples:

Workflow Table of Contents

  1. Report Raster Information – gdalinfo 23
  2. Web Services – Retrieving Rasters (WMS) 29
  3. Report Vector Information – ogrinfo 35
  4. Web Services – Retrieving Vectors (WFS) 45
  5. Translate Rasters – gdal_translate 49
  6. Translate Vectors – ogr2ogr 63
  7. Transform Rasters – gdalwarp 71
  8. Create Raster Overviews – gdaladdo 75
  9. Create Tile Map Structure – gdal2tiles 79
  10. MapServer Raster Tileindex – gdaltindex 85
  11. MapServer Vector Tileindex – ogrtindex 89
  12. Virtual Raster Format – gdalbuildvrt 93
  13. Virtual Vector Format – ogr2vrt 97
  14. Raster Mosaics – gdal_merge 107

Query Vector Data Using a WHERE Clause – ogrinfo

The following is an excerpt from the book: Geospatial Power Tools – Open Source GDAL/OGR Command Line Tools by Tyler Mitchell.  The book is a comprehensive manual as well as a guide to typical data processing workflows, such as the following short sample…

Use SQL Query Syntax with ogrinfo

Use a SQL-style -where clause option to return only the features that meet the expression. In this case, only return the populated places features that meet the criteria of having NAME = ’Shanghai’:

$ ogrinfo 10m_cultural ne_10m_populated_places -where "NAME = 'Shanghai'"

... 
Feature Count: 1 Extent: (-179.589979, -89.982894) - (179.383304, 82.483323) 
... 
OGRFeature(ne_10m_populated_places):6282
 SCALERANK (Integer) = 1 
 NATSCALE (Integer) = 300 
 LABELRANK (Integer) = 1 
 FEATURECLA (String) = Admin-1 capital 
 NAME (String) = Shanghai
... 
 CITYALT (String) = (null) 
 popDiff (Integer) = 1 
 popPerc (Real) = 1.00000000000 
 ls_gross (Integer) = 0 
 POINT (121.434558819820154 31.218398311228327)

Building on the above, you can also query across all available layers, using the -al option and removing the specific layer name. Keep the same -where syntax and it will try to use it on each layer. In cases where a layer does not have the specific attribute, it will tell you, but will continue to process the other layers:

   ERROR 1: 'NAME' not recognised as an available field.

NOTE: More recent versions of ogrinfo appear to not support this and will likely give FAILURE messages instead.

Geospatial Power Tools is 350 pages long – 100 of those pages cover these kinds of workflow topic examples.  Each copy includes a complete (edited!) set of the GDAL/OGR command line documentation as well as the following topics/examples:

Workflow Table of Contents

  1. Report Raster Information – gdalinfo 23
  2. Web Services – Retrieving Rasters (WMS) 29
  3. Report Vector Information – ogrinfo 35
  4. Web Services – Retrieving Vectors (WFS) 45
  5. Translate Rasters – gdal_translate 49
  6. Translate Vectors – ogr2ogr 63
  7. Transform Rasters – gdalwarp 71
  8. Create Raster Overviews – gdaladdo 75
  9. Create Tile Map Structure – gdal2tiles 79
  10. MapServer Raster Tileindex – gdaltindex 85
  11. MapServer Vector Tileindex – ogrtindex 89
  12. Virtual Raster Format – gdalbuildvrt 93
  13. Virtual Vector Format – ogr2vrt 97
  14. Raster Mosaics – gdal_merge 107
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