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I downloaded what I understood to be all of the Ireland OSM data but on examination there seem to be notable differences between the Geofabrik data and what I can see on the OSM map in the browser. The two discrepancy types that I found so far are as follows.
The data I am comparing is on the OSM Ireland map. All the major lakes do not feature as areas in the Geofabrik data (I can see their outline mostly as lines). In the same OSM map, the county boundaries are correct and distinct from any river vector that forms part of that boundary, whereas in the Geofabrik data much of these boundaries are missing. For example, in the GeoFabrik data the Black river forms part of the Galway-Mayo county boundary but this part of the boundary is missing from the Geofabrik data but is not missing in the OSM map on the browser. http://www.openstreetmap.org/?lat=53.47938537597656&lon=-9.161224365234375&zoom=13 Why does the data seem to be different? Is it something to do with licensing? Are those data not open source perhaps? |
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Hi Conor, you have absolutely right. The confusion comes from logical errors in OSM data (dump). Most of the data interpreters and mapping/rendering systems are checking data for formal errors and eventually correct them (like in case of areas, for gaps on the area border polygons, self-crossings on border polygons, orientation and so on). But one huge class of logical errors is rarely (maybe never) checked and corrected. Because these errors are not show stoppers, many rendering systems may live with them. But the displayed/rendered map may vary from case to case and that is what you have discovered in case of the lakes area class for Ireland. For illustration, let us take the natural_water (lakes) area class from the Planet_dump just before the “integer-overflow” (some weeks ago). There are many thousands of replicated lakes. There are cases where the outer/container border polygon is exactly repeated but the hole/island contents are different, cases where some of the border polygons are almost overlapping and so on. Let us take the lake around (-1 064 290; 6 807 950) Mercator (X,Y) point in southwest Ireland, or permalink: http://www.openstreetmap.org/?lat=52.0352&lon=-9.5397&zoom=13&layers=M This lake is repeated once with no holes/islands (L1) and once with 11 islands (L2). Now, if the rendering/mapping system does not render the border polygons in another color you will never see any islands. The same happens if the rendering order is L2, L1. If the rendering order L1, L2 and the border lines are rendered with a color not the same as used for the water, you will see the mentioned strange lines over the water. So, in my opinion, the problem is not the file you have downloaded, it is inherited from the state in the OSM data. Finally, note also that there are many similar cases but much more complex and complicated when replications (overlaps) happen between objects from different classes. For example: when two objects (one from rivers and one from lakes) with different hole contents partly or fully overlap, or when a lake fully or partly overlap some water areas from the planet_land with different hole/island contents and so on. |
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OSM data is being changed or added to continually as you know. At any time someone,such as Geofabrik, may download that current data and render a map in the style of their choice showing or leaving out some stuff. This download and rendering, as its probably done once a week or month and will not include data added after the download. I have not used Geofabrik but would if I wanted other than just the UK. For my Garmin Vista and for Basecamp mapping I use talkytoasters map which also covers Ireland, I like his rendering style it may suit you. |
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So, in the case of the lakes, I will look deeper at the attributes of the lines that make up the boundaries. Yes, my software (Global mapper) does not render on the basis of these other attributes or tags. i uses the area type to render. Also I must look again at the county boundaries. i did see that these lines have an attribute called Admin_Level and that admin_levels 5 and 6 catches most of the boundaries but not all.
(18 Feb '13, 09:43)
TheConor
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I since contacted the developers of Global Mapper GIS software and they were able to make some changes to the software to interpret and render correctly most of the areas (lakes, counties etc.). They said, however... "I checked it looks like the OSM file actually lists the lines that form the boundary of the area in the wrong order, causing the borders to look funny and the areas to be chopped up. I have updated the area building logic to try and look for a connecting path when they aren't specified in an order that connects, which fixes most, but not all of the areas."
(22 Feb '13, 16:27)
TheConor
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It is always "nice" when someone (in this case Global Mapper) tells that something (in this case the OSM data) is broken/wrong, when in fact they simply don't understand it. Boundaries and complex areas are mapped using relations. Nothing in the current data model of OSM requires relations to have a specific order. Ordering the items in a relation is the responsibility of the data consumer.
(22 Feb '13, 23:50)
cartinus
If it is a boundary that still does not look good, then find the ID of the relation (not of one of the ways contained in it) and use the OSM Relation Analyzer to check whether the relation is broken or Global Mapper is mistaken. For complex areas either do the same or look at OSM Inspector's Multipolygon view or Water view
(23 Feb '13, 00:13)
cartinus
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It may be helpful to include URLs to the "Geofabrik data" and "the OSM map" so it is clear what you mean. Please just edit your question to include.
... yes, Conor, please give us some http://wiki.openstreetmap.org/wiki/Permalink to places where you found differences.
...and now we need the filetype of what you downloaded. Was it the shp, the pbf or the osm file from geofabrik?