When you want to buffer features that are spread across a large area (such as global layers), there is no suitable projection that can give you accurate results. This is the classic case for needing Geodesic Buffers – where the distances are measured on an ellipsoid or spherical globe. This post explains the basics of geodesic vs. planar buffers well.Continue reading
You may have seen a map where source and destination points are connected via curved lines. It is possible to create such a map in QGIS with a simple trick – using custom projections and densification of lines. I will outline the steps to create such a map.Continue reading
I got a chance to attend the 3rd Annual QGIS User Conference at A Coruña, Spain.
This was the meeting point of over 100 QGIS developers, users, and trainers from all over the globe. It was the first time I met the QGIS community in person, including some of the people whose work I have admired for years. The event took place over 3 days – 2 days of workshops and 1 day of talks. I am putting some of my notes, takeways and links to resources shared on other channels (twitter, telegram, email) for the benefit of folks who were not present
I recently attended the OpenAQ workshop in Delhi . The workshop’s goal was to bring tech, science and media folks working on air quality together and brainstorm how to use open data to tackle air pollution challenges. Below are my notes and links to materials presented during the workshop.
Below are my notes and links to materials presented during the workshop.Continue reading
This weekend, I got an opportunity to volunteer with a non-profit called Junglescapes. We took a day trip to the Bandipur forest in Karnataka where they have done extensive work in forest restoration. One of their success stories is working with the locals to remove invasive species such as Lantana from the forest. Junglescapes volunteers and locals carry out regular line transact surveys to determine the impact of their interventions. One of the goals for my participation was to see if we can replace the cumbersone paper forms and handheld GPS devices with a mobile-phone based survey using ODK. I am sharing my notes on how we setup the survey and mapping of the result.Continue reading
GeoPDF is a unique data format that brings the portability of PDF to geospatial data. A GeoPDF document can present raster and vector data and preserve the georeference information. This can be a useful format for non-GIS folks to consume GIS data without needing GIS-software. While GeoPDF is a proprietary format, we have a close alternative in the open Geospatial PDF format. GDAL has added support for creating Geospatial PDF documents from version 1.10 onwards. In this post, I will show how to create a GeoPDF document containing multiple vector layers.
Get the Tools
OsGeo4W is the best way to install GDAL on Windows. The default installation gives your GDAL tools with PDF format support. You can use the GDAL tools via the OsGeo4W Shell included in the install.
Once installed, add the path to GDAL library to your
.bash_profile file to be able to use the commands easily from the terminal. Launch a Terminal and type in the following commands.
echo 'export PATH=/Library/Frameworks/GDAL.framework/Programs:$PATH' >> ~/.bash_profile source ~/.bash_profile
Installation instructions will vary with the distribution. On Ubuntu, you can install the
sudo apt-get install gdal-bin
Verify GDAL Install
If you already have GDAL installed, or just installed it, run the following command in a terminal to verify that your GDAL installation is working and has support for GeoPDF format.
gdalinfo --formats | grep -i pdf
If you see Geospatial PDF printed in the output – you are all set. If you do not get any output or get an error, your install is not correctly configured.
Get the Data
For this example, I chose to use OpenStreetMap Metro Extracts from MapZen. Download the shapefiles (OSM2PGSQL SHP format) for the city of your choice. I am using the extract for Bangalore city in this example. Unzip the downloaded file to a folder on your computer.
The process for creating a GeoPDF file from a bunch of shapefiles is the matter of running a single
gdal_translatecommand. But we need to prepare the data and figure out the correct command-line options. So follow along to understand how you can arrive at the final command – or simply scroll to the end to see the final command-line.
latuviitta.org has a comprehensive overview of all the options available for GeoPDF creation via GDAL. The follow steps are adapted and simplified version of that guide.
- First step is to create a
.vrtfile that can hold all the vector layers we want in the PDF. If you just need a single layer in the PDF, you can skip creating the
.vrtfile and directly reference the layer in place of the VRT. Note the <SrcSQL> tag in the VRT file. This is for filtering out all features where the ‘name’ field is empty. You can leave that out or modify to suit your dataset. Name this file
osm.vrtand save it on the same folder with your data.
<OGRVRTDataSource> <OGRVRTLayer name="roads"> <SrcDataSource>bengaluru_india_osm_line.shp</SrcDataSource> <SrcLayer>bengaluru_india_osm_line</SrcLayer> <SrcSQL dialect="sqlite">SELECT name, highway, geometry from bengaluru_india_osm_line where name is not NULL</SrcSQL> <GeometryType>wkbLineString</GeometryType> <LayerSRS>WGS84</LayerSRS> </OGRVRTLayer> <OGRVRTLayer name="pois"> <SrcDataSource>bengaluru_india_osm_point.shp</SrcDataSource> <SrcLayer>bengaluru_india_osm_point</SrcLayer> <SrcSQL dialect="sqlite">SELECT name, geometry from bengaluru_india_osm_point where name is not NULL</SrcSQL> <GeometryType>wkbPoint</GeometryType> <LayerSRS>WGS84</LayerSRS> </OGRVRTLayer> </OGRVRTDataSource>
- GeoPDF is a raster format that can overlay vectors on top. So we need a raster layer as the base. If you have some satellite imagery or scanned raster for the area, you can use it as the base layer, or we can create an empty raster for the extent of the vector layer.
ogrtindexcommand creates a bounding box polygon from the given input layers.
gdal_rasterizecommand then fills this polygon with the given value and creates a raster. The
-troption specifies the pixel resolution of the raster in degrees. You can tweak that to get the output size you need.
cdto the directory where you have extracted the vector layers and run the following commands.
cd Users\Ujaval\Downloads\bengaluru_india.osm2pgsql-shapefiles ogrtindex -accept _different_schemas extent.shp osm.vrt gdal_rasterize -burn 255 -ot Byte -tr 0.0001 0.0001 extent.shp bangalore.tif
- Now we can convert the empty
bangalore.tifraster to a PDF – overlaying the vector layers from the
gdal_translate -of PDF -a_srs EPSG:4326 bangalore.tif bangalore.pdf -co OGR_DATASOURCE=osm.vrt -co OGR_DISPLAY_FIELD="name"
- Once the conversion finishes, you can open the resulting
bangalore.pdffile in any PDF viewer. Opening it in Adobe Acrobat viewer, you can see the map data layers. You can browse the features in the layer panel, search for any attribute value and zoom/pan the map.
- Another popular use of GeoPDF files is to use it as offline base maps using programs such as Avenza PDF Maps. Loading the
bangalore.pdffile on Avenza Maps on your mobile phone, you can use the GPS to view your current location or trace a GPS route on top. Search also works across layers in the PDF.
You can download the sample bangalore.pdf Geospatial PDF format file for exploring the format yourself.
Mapshaper is a free and open-source tool that is best known for fast and easy simplification. Other tools for simplification – like QGIS or ogr2ogr – do not preserve topology while simplifying. This means you may get sliver polygons or missing intersections. Mapshaper performs topologically-aware simplification and gives you much more control on the process.Continue reading
GDAL and OGR libraries come with handy command-line tools. These tools are quite powerful and can save you a lot of effort if you know how to use them. Here I will show how to use the ogrinfo and ogr2ogr tools to perform spatial joins. A single command can do complex operations on your spatial data and save you a lot of clicking-around and data-munging in a GIS.
Get the Tools
Get the Data
Review the data and problem statement from the Performing Spatial Joins tutorial. Download the Borough Boundaries and Nursing Homes shapefiles.
OGR command line tools accept only 1 input. But we have 2 inputs for the spatial join. An easy way to fix this, is to use a VRT file. A VRT file allows us to specify multiple inputs and pass them to the command-line tool as layers of a single input.
Unzip the input shapefiles in a single folder on your drive. Create a file named input.vrt in the same folder with the following content.
<OGRVRTDataSource> <OGRVRTLayer name="boroughs"> <SrcDataSource>nybb.shp</SrcDataSource> <SrcLayer>nybb</SrcLayer> </OGRVRTLayer> <OGRVRTLayer name="nursinghomes"> <SrcDataSource>OEM_NursingHomes_001.shp</SrcDataSource> <SrcLayer>OEM_NursingHomes_001</SrcLayer> </OGRVRTLayer> </OGRVRTDataSource>
Open the OSGeo4W shell and cd to the directory containing the shapefiles and the vrt file. Run the ogrinfo command to check if the VRT file is correct.
OGR tools can run SQL queries on the input layers. We will use the ST_INTERSECTS function to find all nursing homes that intersect the boundary of a borough and use the SUM function to find the total nursing home capacity of a borough. Run the following command.
ogrinfo -sql "SELECT b.BoroName, sum(n.Capacity) as total_capacity from boroughs b, nursinghomes n WHERE ST_INTERSECTS(b.geometry, n.geometry) group by b.BoroName" -dialect SQLITE input.vrt
You can see that in a single command we got the results by doing a spatial join that takes a lot of clicking around in a GIS environment. We can do a reverse spatial join as well. We can join the name of the Borough to each feature of the Nursing Homes layer. Using the ogr2ogr tool we can write out a shapefile from the resulting join. Note that we are adding a geometry column in the SELECT statement which results in a spatial output. Run the following command:
ogr2ogr -sql "SELECT n.Name, n.Capacity, n.geometry, b.BoroName from boroughs b, nursinghomes n WHERE ST_INTERSECTS(b.geometry, n.geometry)" -dialect SQLITE output.shp input.vrt
Open the output.shp in a GIS to verify that the new shapefile as attributes joined from the intersecting borough. You can use ogrinfo command to check that as well.
ogrinfo -al output.shp
The Association for People with Disability (APD) is a non-profit organization based out of Bangalore, India. Their mission is to reach out and rehabilitate people with disability from the under privileged segment. Over the past year, I along with my colleagues have been volunteering with them to develop a system that can help improve their field data collection efforts.
APD provides variety of services rehabilitate under privileged people with disabilities. Before they can render any service, they need register the individuals with the organization and collect basic background information. Registrations are mostly done in their field offices or at camps organized in rural areas. Paper forms were filled at the site and shipped to their field office. A staff member entered the data manually into their software platform.
This has many problems:
- The text on the paper form was often illegible. Some fields were missing or inaccurate.
- Data entry was laborious and introduced errors.
- 4–6 weeks of lag time before the data was available in the system.
We helped APD implement a process using OpenDataKit (ODK) that allowed capture of the form using android devices. With the new system, the data is captured on the mobile device using the ODK Collect app in the field and sent to a ODK Aggregate server running on Google AppEngine. The data ends up in a shared spreadsheet which is imported to APD’s system after each registration camp.
This new workflow offers several advantages over the paper based forms:
- Reliable data collection in areas with poor network connectivity. ODK Collect app can work completely offline and the data is stored on the device memory. Once the staff members are back in office and connect to a WiFi network — the data is sent to the server.
- The mobile app enforces checks, so all the data is consistent and there are no missing fields.
- Allows for the capture of pictures and additional metadata (such as time, location, staff id).
- The data is exported from the spreadsheet and imported to their system the next day — cutting the lag from weeks to hours.
- October 2014: Met with the APD registration team to understand their requirements and design a process.
- November 2014: A prototype is created by migrating the registration form to ODK using XLSForm. APD trains field staff on using the mobile app. Successful field test.
- December 2014: First full-fledged camp with 3 devices. Successful registration of 55 participants. Staff is very happy with the increased speed, accuracy and reduction in delay in getting the registrations processed.
- January 2014: APD moves their registrations completely to mobile devices. All registrations are completely paperless and have the added benefit of having participant’s picture as part of the registration process. Over 500 registrations processed without a problem.
While this is a great start, we are looking at helping them with other challenges in the field. In the coming months, we want to tackle the following problems:
- Migrate patient visit and treatment forms to OpenDataKit. These require having access to the patient’s medical history in the field. OpenDataKit’s 2.0 suite of tools would be a good fit.
- Task allocation and scheduling optimization for the field staff.
- Encoding the knowledge from the training manual to a mobile app.