PROJ 4.8.0 Released

Early this week I announced the release of PROJ 4.8.0.  It turns out the last release (4.7.0) was in 2009 so it has been quite some time coming. 

PROJ.4 is a C library implementation transformations between a wide variety of geospatial coordinate systems.  It features include support for many projection methods (ie. Transverse Mercator) as well as support for converting between geodetic datums.  As a library it is used by many of the C/C++ open source geospatial applications, and a few proprietary ones including GDAL/OGR, PostGIS, GRASS, MapServer and QGIS (some of my favorites).  It also includes commandline utilities mostly used for testing and a dictionary of coordinate system definitions based on the EPSG coordinate system from OGP.

The announcement lists the major new features, but there is a subset I would like to call out here because I think they are quite significant.

1. A great deal of work went into making the library threadsafe.  The main previous hole in threadsafe operation was the use of the global pj_errno for errors.  This has been worked around by introduction of an "execution context" object (projCtx) which can be passed in when creating projection objects (projPJ).  The execution context also now provides a mechanism to manage error and debug message output. See use of projCtx in proj_api.h for an idea of how this would be used.  The execution context also means that more than one part of a process can use PROJ.4 with less risk of stepping on each other (imagine MapServer and GDAL/OGR for instance). 
2. Preliminary support has been added for vertical datums using the +geoidgrids= directive.  The geoidgrids are assumed to provide a transformation from geoid elevations to ellipsoidal elevations and they are discussed a bit in the VerticalDatums topic on the web site.
3. I have introduced yet another supported horizontal grid shift file format which I call CTable2.  It is really the same as the venerable binary format produced by nad2bin, but with the header regularized and the byte order fixed so that the format is binary compatible across all platforms.  This means it is no longer to worry about having distinct copies of the grid shift files for environments like 32bit vs. 64bit linux, or windows vs. linux.  I had originally intended to just migrate to using NTv2 but it turns out to take twice as much space because of the presence of rarely used error estimate bands. 
4. The addition of the +axis= switch to control axis orientation.  This makes it possible to handle coordinate systems where "X" is north and "Y" is east as well as various other combinations.  

On the one hand there is relatively new in the coordinate of geospatial coordinate systems as far as "discovers" go.  But in practice it is becoming expected that quality geospatial software will support the full suite of EPSG coordinate systems which requires better handling of units, axes, datum shifts, prime meridians and vertical coordinate systems.  I think PROJ.4, a core piece of technology in the FOSS4G stack, has made substantial progress over the last couple of years. 

I would also be negligent if I failed to call out the important funding support of the US Army Corp of Engineers in these developments via Howard Butler and Liblas / PDAL.

FOSS4G Videos

Andrew Ross has announced that all the FOSS4G 2011 and State of the Map videos are now up and available at the FOSSLC web site. Andrew provided the following urls to scan these two categories which gives an extended blurb about each video:

http://www.fosslc.org/drupal/category/event/foss4g2011
http://www.fosslc.org/drupal/category/event/sotm

However I prefer the search mechanism which gives lots more talks per page with just the title.

http://www.fosslc.org/drupal/search/node/foss4g2011

At the risk of being self obsessed I'll highlight two of my talks. I can't seem to find the other two.

• My Life as an Open Source Developer
• Tuning GDAL Raster Performance

I had also really wanted to attend Martin Davis' talk on Spatial Processing Using JEQL but somehow missed it. This weekend I'll review it. There were some other great keynotes and presentations. As you can imagine Paul Ramsey's keynote on Open Source Business Models was informative, amusing and engaging.

The videos were captured by Andrew's team of volunteers, and deployed on the FOSSLC site which includes lots of great videos of talks from other conferences. This has really be a passion of Andrew's for some time and at least in the OSGeo community I don't think the value of it has been fully recognized and exploited. I hope that others will look through the presentation and blog, tweet, +1 and like some that they find the most useful and interesting. Virtual word of mouth is one way to improve the leverage from the work of capturing the videos and the effort to produce the original presentations.

I'd also like to thank Andrew and his helpers Teresa Baldwin, Scott Mitchell, Christine Richard, Alex Trofast, Thanha Ha, "Steve", Paul Deshamps, Assefa Yewondwossen, Nathalie West, Mike Adair and "Ben", many of whom I count as personal friends from the OSGeo Ottawa crowd.

Joining Google

Today I accepted a job with Google as a GIS Data Engineer. I will be based in Mountain View California at head office, and involved in various sorts of geodata processing though I don't really know the details of my responsibilities yet.

I have received occasional email solicitations from Google recruiters in the past, but hadn't really taken them too seriously. I assumed it was a "wide net" search for job candidates and I wasn't looking for regular employment anyways. I was happy enough in my role of independent consultant. It gave me great flexibility and a quite satisfactory income.

Various things conspired to make me contemplate my options. A friend, Phil Vachon, moved to New York City for a fantastic new job. His description of the compensation package made me realize opportunities might exist that I was missing out on. Also, with separation from my wife last year the need to stay in Canada was reduced. So when a Google recruiter contacted me again this spring I took a moment to look over the job description.

The description was for the role of GIS Data Engineer it was a dead on match for my skill set. So, I thought I would at least investigate a bit, and responded. I didn't hear back for many weeks so I assumed I had been winnowed out already or perhaps the position had already been filled. But a few weeks ago I was contacted, and invited to participate in a phone screening interview. That went well, and among other things my knowledge of GIS file formats, and coordinate systems stood me in good stead. So I was invited to California for in person interviews.

That was a grueling day. Five interviews including lots of "write a program to do this on the black board" sorts of questions. Those interviewing me, mostly engineers in the geo group, seemed to know little of my background and I came away feeling somewhat out of place, and like they were really just looking for an entry level engineer. Luckily Michael Weiss-Malik, spent some time at lunch talking about what his group does and gave me a sense of where I would fit in. This made me more comfortable.

Last week I received and offer and it was quite generous. It certainly put my annual consulting income to shame. Also contact with a couple friends inside Google gave me a sense that there were those advocating on my behalf who did know more about my strengths.

I agonized over the weekend and went back and forth quite a bit on the whole prospect. While the financial offer was very good, I didn't particularly need that. And returning to "regular employment" was not inline with my hopes to travel widely, doing my consulting from a variety of exotic locales as long as they had internet access. But, by salting away money it would make it much easier to pursue such a lifestyle at some point in the future.

I also needed to consult extensively with my family. It was very difficult to leave my kids behind, though it helps to know that as teens they are quite independent, I am also confident that their mother would be right there taking good care of them. My kids, and other family members were all very supportive.

My other big concern was giving up my role in the open source geospatial community. While nothing in the job description prevents me from still being active in the GDAL project, and OSGeo, it is clear the job will consume much of my energy and time. I can't expect to play the role I have played in the past, particularly of enabling use of GDAL in commercial contexts by virtual of being available as a paid resource to support users.

I don't have the whole answer to this, but it is certainly my intention to remain active in GDAL/OGR and in OSGeo (on the board and other committees). One big plus of working at Google is the concept of 20% time. I haven't gotten all the details, but it is roughly allowing me to use 20% of my work time for self directed projects. My hope is to use much of my 20% time to work on GDAL/OGR.

Google does make use of GDAL/OGR for some internal data processing and in products like Google Earth Professional. My original hope had been that my job would at least partly be in support of GDAL and possibly other open source technologies within Google. While things are still a bit vague that does not seem to be immediately the case though I'm optimistic such opportunities might arise in the future. But I think this usage does mean that work on GDAL is a reasonable thing to spend 20% time on.

I also made it clear that I still planned to participate in OSGeo events like the FOSS4G conference. I'm pleased to confirm that I will be attending FOSS4G 2011 in Denver as a Google representative and I am confident this will be possible in future years as well.

The coming months will involve many changes for me, and I certainly have a great deal to learn to make myself effective as an employee of Google. But I am optimistic that this will be a job and work place that still allows me to participate in, and contribute to, the community that I love so much. I also think this will be a great opportunity for me to grow. Writing file translators for 20 years can in some ways become a rut!

MapServer TIFF Overview Performance

Last week Thomas Bonfort, MapServer polymath contacted me with some surprising performance results he was getting while comparing two ways of handling image overviews with MapServer.

In one case he had a single GeoTIFF file with overviews built by GDAL's gdaladdo utility in a separate .ovr file. Only one MapServer LAYER was used to refer to this image. In the second case he used a distinct TIFF file for each power-of-two overview level, and a distinct LAYER with minscale/maxscale settings for each file. So in the first case it was up to GDAL to select the optimum overview level out of a merged file, while in the second case MapServer decided which overview to use via the layer scale selection logic.

He was seeing performance using the MapServer multi-layer approach being about twice what it was with letting GDAL select the right overview. He prepared a graph and scripts with everything I needed to reproduce his results.



I was able to reproduce similar results at my end, and set to work trying to analyse what was going on. The scripts used apache-bench to run MapServer in it's usual cgi-bin incarnation. This is a typical use case, and in aggregate shows the performance. However, my usual technique for investigating performance bottlenecks is very low-tech. I do a long run demonstrating the issue in gdb, and hit cntl-c frequently, and use "where" to examine what is going on. If the bottleneck is dramatic enough this is usually informative. But I could not apply this to the very very short running cgi-bin processes.

So I set out to establish a similar work load in a single long running MapScript application. In doing so I discovered a few things. First, that there was no way to easily load the OWSRequest parameters from an url except through the QUERY_STRING environment variable. So I extended mapscript/swiginc/owsrequest.i to have a loadParamsFromURL() method. My test script then looked like:

import mapscript

map = mapscript.mapObj( 'cnes.map')
mapscript.msIO_installStdoutToBuffer()

for i in range(1000):
  req = mapscript.OWSRequest()
  req.loadParamsFromURL( 'LAYERS=truemarble-gdal&FORMAT=image/jpeg&SERVICE=WMS&VERSION=1.1.1&REQUEST=GetMap&STYLES=&EXCEPTIONS=application/vnd.ogc.se_inimage&SRS=EPSG%3A900913&BBOX=1663269.7343875,1203424.5723063,1673053.6740063,1213208.511925&WIDTH=256&HEIGHT=256')
  map.OWSDispatch( req )

The second thing I learned is that Thomas' recent work to directly use libjpeg and libpng for output in MapServer had not honoured the msIO_ IO redirection mechanism needed for the above. I fixed that too.

This gave me a proces that would run for a while and that I could debug with gdb. A sampling of "what is going on" showed that much of the time was being spent loading TIFF tags from directories - particularly the tile offset and tile size tag values.

The base file used is 130000 x 150000 pixels, and is internally broken up into nearly 900000 256x256 tiles. Any map request would only use a few tiles but the array of pointers to tiles and their sizes for just the base image amounted to approximately 14MB. So in order to get about 100K of imagery out of the files we were also reading at least 14MB of tile index/sizes.

The GDAL overview case was worse than the MapServer overview case because when opening the file GDAL scans through all the overview directories to identify what overviews are available. This means we have to load the tile offset/size values for all the overviews regardless of whether we will use them later. When the offset/size values are read to scan a directory they are subsequently discarded when the next directory is read. So in cases where we need to come back to a particular overview level we still have to reload the offsets and sizes.

For several years I have had some concerns about the efficiency of files with large tile offset/size arrays, and with the cost of jumping back and forth between different overviews with GDAL. This case highlighted the issue. It also suggested an obvious optimization - to avoid loading the tile offset and size values until we actually need them. If we access an image directory (ie overview level) to get general information about it such as the size, but we don't actually need imagery we could completely skip reading the offsets and sizes.

So I set about implementing this in libtiff. It is helpful being a core maintainer of libtiff as well as GDAL and Mapserver. :-) The change was a bit messy, and also it seemed a bit risky due to how libtiff handled directory tags. So I treat the change as as an experimental (default off) build time option controlled by the new libtiff configure option --enable-defer-strile-load.

With the change in place, I now get comparable results using the two different overview strategies. Yipee! Pleased with this result I have commited the changes in libtiff CVS head, and pulled them downstream into the "built in" copy of libtiff in GDAL.

However, it occurs to me that there is still an unfortunate amount of work being done to load the tile offset/size vectors when doing full resolution image accesses for MapServer. A format that computed the tile offset and size instead of storing all the offsets and sizes explicitly might do noticeable better for this use case. In fact, Erdas Imagine (with a .ige spill file) is such a format. Perhaps tonight I can compare to that and contemplate a specialized form of uncompressed TIFF files which doesn't need to store and load the tile offset/sizes.

I would like to thank Thomas Bonfort for identifying this issue, and providing such an easy to use set of scripts to reproduce and demonstrate the problem. I would also like to thank MapGears and USACE for supporting my time on this and other MapServer raster problems.

RPATH, RUNPATH and LD_LIBRARY_PATH

Long time since my last post - my bad.

Recently I have been working on a renewed FWTools for linux - hopefully I'll follow up on that later. Much like in the past FWTools is intended to work across a variety of linux systems (ie. Ubuntu, Redhat, Suse, Gentoo) and to not interfere with the native packing system nor to require special permissions to install.

To that end we try to distribute as many of the required shared libraries as possible, and we use the LD_LIBRARY_PATH environment variable to ensure FWTools finds and uses these shared libraries in preference to any system provided versions which might be incompatible (though often aren't).

In my local testing I was getting frustrated as my shared libraries were not being used. Instead the system versions of things like libsqlite3.so.4 were getting picked up from /usr/lib64 despite my LD_LIBRARY_PATH pointing to a copy distributed with FWTools. I check this with the ldd command which shows what sublibraries an executable or shared library need and which is being picked according to the current policy.

I carefully read the man page for ld.so, which is responsible for loading shared libraries at runtime, and it indicated:

The necessary shared libraries needed by the program are searched for
in the following order

o Using the environment variable LD_LIBRARY_PATH
(LD_AOUT_LIBRARY_PATH for a.out programs). Except if the exe‐
cutable is a setuid/setgid binary, in which case it is ignored.
...

Well, my programs are not setuid/setgid nor are they a.out (very old style of linux binary), so what gives? Due to poor time synchronization on my systems and use of nfs, I noticed that some shared library dates were in the future. I thought perhaps some funky security policy prevented them from being used, but that theory did not pan out. I tried doing it on a local file system incase there were some rule about not trusting shared libraries from remote file systems in some cases. Still no go.

However, googling around the net I found that my local ld.so man page was not very comprehensive. Another ld.so man page gave me a hint, referring to DT_RPATH and DT_RUNPATH attributes being used before LD_LIBRARY_PATH. I was vaguely aware of shared libraries having an "rpath" attribute which could be set at link time and might have some sort of influence on searching. Distribution packaging folks have sometimes bugged me about it, especially back in the old days before libtool was used for GDAL. But I had never really understood it well and it just seemed like unnecessary stuff.

Further digging confirmed that ELF (the modern linux object file format) executables and shared libraries may have an RPATH attribute built right into the file. This is essentially a directory that should be searched first for shared libraries that this executable or shared library depends on. RUNPATH is similar, but is apparently evaluated after LD_LIBRARY_PATH. I got my critical information from this blog post by Trevor King.

My old favorite, the ldd command, does not show the rpath settings though it does honour them. I also use nm to dump the symbols in object files and shared libraries, but it doesn't address special attributes like rpath. Digging around I found the readelf command which can report all sorts of information about a shared library or executable, including the RPATH, and RUNPATH. It also helpfully lists the shared libraries that are needed, something that often gets obscured in ldd by the chaining of dependencies.

The rpath attribute is generally set by libtool when linking shared libraries. GDAL includes a build option to build without libtool, and I contemplated using that which would help alot. But it wouldn't help for other libraries in FWTools that were not directly pulled in by GDAL. My goal with the modern FWTools is to - as much as possible - use standard CentOS/RedHat rpm binaries as the basis for the product. So I don't want to have to rebuild everything from scratch. What I really wanted was a way to suppress use of the rpath, switch it to runpath or remove it entirely. Low and behold, I am not the first with this requirement. There is a command called chrpath that can do any of these "in place" to a binary (see chrpath(1) man page).

chrpath -d usr/lib64/libgdal.so

This command isn't installed by default on my system, but it was packaged and it works very nicely. I'll preprocess the shared libraries and binaries to strip out rpath so my LD_LIBRARY_PATH magic works smoothly. I had even considered using it to replace the rpath at install time with the path inside the FWTools distribution for libraries. This, unfortunately, does not appear to be possible as it seems chrpath can only replace rpath's with rpaths of the same or a shorter length. In any event, FWTools still depends on cover scripts to set other environment variables so doing this wouldn't have made it practical to avoid my environment setup magic.

I owe a lot to some other blog and mailing list posts turned up by google. This blog entry is partly an attempt to seed the net with helpful answers for others that run into similar problems. I will say it is very annoying that a company decided to name itself "rPath" - it made searching quite a bit harder!

OGR DXF Upgrade

Well, I just finished up a week of work implementing PCIDSK Vector write and update support. My next task is an upgrade to the OGR DXF driver requested by Stadt Uster to better meet their production requirements.

They need the ability to control layer naming, production of dash patterns, and producing objects as block references.

Currently the DXF writer is dependent on everything except the entities section coming directly from a template file. This has meant it was not practical to create layers, line styles and block references on the fly. This is going to have to change now, though we will continue to use the template header extensively.

The first approach considered was just to require the user to develop a template header with all the layer names, line styles and block references predefined. This might have been adequate for Uster who have specific needs and once a template was prepared they could generally just reuse it for additional products. But it would have made the new capabilities of very little utility to other users of GDAL.

So the planned approach has two parts. First we scan the template header for layer definitions, and block definitions (and possibly line style definitions). Then as we go through the entities if we find these layers or blocks referenced we just use them directly.

However, if we find layers referenced from the objects being written to the DXF file (based on the "Layer" attribute) we will automatically create a new layer in the header, matching the configuration of the default layer ("0"). This means we can automatically create layers that have identity even if they are otherwise indistinguishable.

For block references we use a roughly similar approach. We prescan for block definitions, but then we extend them with any entities written to an OGR layer named "Blocks". Normally all DXF entities are exposed through an OGR layer called "entities" though when writing we accept any layer name, except now for Blocks which is special. Then when we write we allow these blocks to be referenced based on a BlockName attribute.

Corresponding behavior will also be available when reading. If desired (a config option turned on) we will expose block definitions as a Blocks layer, and the actual block references in the entities layer will just be a point feature with insertion information. The default behavior will remain what it does not - which is to inline copies of the block geometries for each block reference as this is the only approach that will be handled gracefully by most applications or writers.

For line styles it is not clear that it is helpful to predefine them (though I'll examine that during implementation). The only aspect we are interested in preserving and producing is dash-dot patterns. But while writing I will create new line styles for each such pattern encountered, and then write them out to the header.

Currently the DXF writer copies the template header to the output file, then writes entities, then appends the trailer template. In the future I will need to write the entities to a temporary file, storing up block, layer and line style definitions in memory. Then on closing the dataset I will need to compose the full header, append the entities and trailer.

I dislike this pattern. It introduces a need for temporary files which can lead to surprising disk use requirements. Also, if something goes wrong the temp files may not get cleaned up properly. We also lose any hope of streaming operation. However, to achieve what we want to with the DXF driver it seems unavoidable.

Hopefully I'll have the DXF changes in trunk within a couple weeks. If there are folks interested in DXF generation, keep an eye on SVN for updates. Beta testers appreciated!

Travelling

I have spent this week in beautiful Chicoutimi / Saguenay this week at the Rendez-vous OSGeo Québec. This is a two day conference put on by the local chapter here in Quebec in association with another local geomatics conference.

It has been some time since I have been to a significant event, unfortunately having missed FOSS4G in Sydney, so it was good to be involved again. As long ago as the Ottawa OSGIS conference in 2004 I recall Daniel Morissette, Steve Lime, and me hanging out in Dave McIllhagga's penthouse and talking about visiting Chicoutimi. Perhaps because of that Daniel, who was a primary organizer, made an effort to invite Steve and me. Unfortunately Steve was unable to attend in person due to a knee injury, but many other friends were here including Paul Ramsey and Jeff McKenna. As you can imagine, in addition to an excellent program, there were great social activities.

Today, during the other geomatics conference, I am helping to man the OSGeo booth. I unfortunately speak no french, and I was worried about having nothing I could say or provide to french-only speakers. However, I was immensely pleased to discover that the booth was well equipted with the full set of OSGeo promotional materials all translated into french, printed out and very professional looking.

I have at times not been all that excited about translation efforts. I am uni-lingual, and can't really help. I have also had the impression that at least some of the translation efforts have been rather haphazard and might not reflected particularly well on the projects or OSGeo. But here I was very impressed with the complete and professional job that has been done with the promotional materials, and also how useful it is. It really is very helpful for our credibility today, and I can see this would apply in many other venues. I understand the translation is done by the Francophone chapter, and that one major contributor has been Yves Jacolin. I am sure there are many others also involved. So, my hat is off to their efforts.

I should also mention, I finally got to see Paul's fantastic talk "Beyond Nerds Bearing Gifts: The future of the Open Source Economy". I can see why everyone was raving about it after Sydney. One message that I took away from the talk was that as all companies become open source companies to some degree, we need to be cautious about thinking of ourselves primarily as open source companies or consultants. Our excellent integration into the open source ecosystem is one of our selling points, but we need to ensure our primary focus is on serving the needs of our clients effectively and efficiently.

Today I was also invited to attend a technical meeting in Zurich at the end of the month as part of the GEM (Global Earthquate Model) project. I was not aware of this activity at all until Steve Arnold (aka nerdboy on IRC) brought it to my attention recently. Among other things, GEM is interested in using open source components in their IT infrastructure and I will provide some feedback on their plans and efforts so far, particularly on the geospatial aspects.

This week, after discussions with Paul Ramsey, Jeff McKenna, and Venkatesh Raghavan, I am tentatively planning to go to Japan in November for the OSGeo meetings held there. Everyone who has attended these meetings in the past as an invited speaker has raved about what a good experience it is, so I am looking forward to it very much. I believe that Paul Ramsey will also likely be there, and Jeff may be back in Japan at the same time for his OSGeo4W cooperation.

Also, everyone is very much looking forward to Barcelona. I am confident it will be the best FOSS4G ever. I have registered, but still need to book my flight and hotel. I'm thinking of taking a little extra time to see the area. I sometimes find travel tiring and puts me behind in my work. But I think the social and networking opportunities are too good to miss. These meetings reengerize me in ways that last much longer than the jet lag.