Monday 31 October 2011

Wifi coverage maps

I have been working on a small project for a Community Interest Company who deliver Fi-Wi and who want to display the coverage area of their wifi. They want to show the coverage as bands for their speed of connection: 10Mb, 20Mb, 30Mb etc. They cover a number of areas across the country, especially rural areas. I needed to display a polygon over an OSM map. Their website already has a map on it, using Cloudmade tiles, but with no coverage area on it yet. I took a look at GeoJSON as a way to handle description of polygons and looked at the way OpenLayers handles it. I created a GeoJSON feature collection by hand and tried to display it with OpenLayers. After a frustrating couple of hours I hadn't persuaded it to display my polygons, so I turned to Leaflet. In a few minutes, starting from scratch, I had a map with my polygon on it.

I needed to manage many polygons so I created an AJaX feed to send a GeoJSON feature collection for a bounds box. That way when a map is displayed any of the polygons that should appear on it get sent for that map. The coverage polygons are stored in a database with their maximum extents for north, south, east and west which can them be used to decide which polygons are served for the current bounds box of the displayed map. As usual AJaX is quick and effective. Debugging is fairly easy too since a call to the AJaX server returns a GeoJSON file which is easy to read.

The remaining job was to create a polygon editor so the guys who manage the website can manage the shape of the polygons easily as well as adding new ones as their business expands. This was more intricate, though I'm not sure that I have yet done as much as I would like.

I've enjoyed working with this problem, I hope it helps the company. It has also convinced me that Leaflet is a much more pleasant way to display maps with overlays than wrangling OpenLayers, which may be powerful and very flexible but it always causes me problems.

Saturday 29 October 2011

Chasing names

There is a list of roads sorted by county that compares the names on OSM maps to the names on the OS OpenData Locator. Chasing the names to get 100% is a bit phoney, but using OS Locator to find what is missing and especially if something has been added since the last OSM survey is very good. However much I think the name chase is phoney or even invites people to add stuff to OSM that isn't correct or surveyed, to make use of the idea that OS Locator help find new stuff, first you have to get a clean state to work from. The simplest clean state is to survey all of the roads that show up as anomalies and resolve the anomalies. In the process you see more details to add and things that have changed, such as closed pubs. I only add what I have seen for myself, though I will use aerial imagery and OpenData to assist with things like just what is the extent of that playing field that I now have the name of and can see is in use. Using OS Locator names for roads where the name board is not present is sometimes an exception, in that I have not seen the name, but I use it anyway. I do of course credit OS with a source:name tag.

Today we focussed on the southern part of the Holderness plain in East Yorkshire. It is a fairly flat area, with fertile aluvial soils making farming productive. The southern edge borders the Humber estuary which is miles wide at this point. Today the tide was out and the huge expanses of mud flats were exposed, which is what the wading birds like to feed on. The wild life is worth looking out for. It was a lovely place to take a break.

My aim is to clear Holderness steadily from the south, working past Withernsea towards Hornsea. I was contacted in the week by Stephen who mapped the whole of the town of Withernsea and then went quiet. He has now accepted the new licence which is very welcome; instead of starting again with Withernsea we can build on his good work and concentrate of places that need surveys, like Hornsea.

Saturday 22 October 2011


A bright, fairly sharp day tempted us out. I set off to check a few names in the Wolds village of Kilham, and in the end just about redrew it. The existing work was by a few people who had added bits that didn't join up, were very, very, approximate and had loads and loads missing. Most road names were missing even though there were clear names on every road. There was a name issue near the school. One end of the road has a board showing "Millside", the other "Mill Side". There is a change of speed limit on the road, so each end gets the respective name.  It took a while, but it was a pleasant place to be. One thing us OSMers like is to find a substantial road that is missing. Seeing that GPS trace meander across an empty space is satisfying and today it happened twice for country roads, once where a rural road was marked as a bridleway and lots of times for small residential roads.

I usually draw areas such as sports grounds and school fields from Bing imagery when I can, but a very new recreation ground was just a field on Bing. I had to guess how much of the field was taken. There was also a short road with a few brand new, empty bungalows on it. The road has no name board and doesn't appear on OS Locator, so I left a fixme for the name.

On the way home I we passed some road works, with a footpath / cycle lane next to the road. There was a sign giving orders to cyclists. Clearly the person who erected the sign doesn't know the rules.

Tuesday 4 October 2011

The end of the strangely warm start to October saw us out onto the Holderness Plain looking for names of roads and any other stuff worth improving. Once again we discovered a road name that Ordnance Survey have just made up without checking what is really there. There is a road called Fox Covert Road just north of Otteringham. There is a sign which is not new, but is perfectly readable.

OS call the road Whinhill Road. The road is a minor lane which only goes to a couple of farms, the second of which is called Whinhill farm, so I guess that's where OS get the name from. Nearby OS show a road called North End road and there is a farm called North End Farm too, this time no sign boards to confirm or deny this. The road from Otteringham is called Station Road because it headed to the abandoned station on the Withernsea line, which fell under Dr Beeching's axe. The place where the name changes to North End Road is not clear, but near the farm seems a good idea. I suspect the road may have been called North End Road long before part of it was called Station Road, but I can't be sure.