## 3 Results and Discussion
### Collaborative Mapping
After a manual calibration of the laser telemeter data, GPS data  
Figure 10: **(a)** Volunte as well as professional geographer [PERSON] with indoor laser telemeter; **(b)** Advanced field equipment for editing and upload of data on-site. Figure 9: **(a)** Before: unmapped and untraceable GK village _Telus_. **(b)** After: uploaded mapping (screenshot osm.org, Feb 1, 2010) with samples of building footprints ([PERSON]). and the paper maps, they generated an on-screen fair drafting in JOSM, added useful attributes and were ready to upload the new map tiles. The principle of gathering, processing and upload of all data on one day and on-site, offers an essential advantage: Mistakes or doubts can immediately be checked _in situ_. _MotionX GPS_ is a powerful App, but the power consumption of the built-in iPhone GPS receiver is high and sub-street-level accuracy of such standard GPS chips in \"urban canyons\" is low. The segmentation and narrowness of the informal city does not correspond to such a GPS signal variance and vice versa. To get more than 8-10 m accuracy we would have to move to a differential GPS system. The Filipino OSM volunteers reported that they sometimes use an external antenna in the urban context, mounting it on a bamboo stick to receive better GPS signals. When mapping in areas of high housing density, they recommended setting some reference points in open areas adjacent to these density zones in order to process data with higher accuracy. Elevation measurement from the iPhone 4 was frequently found unreliable and imprecise, _i.e._, useless. _Telus_ village does not have street names so far, but house numbers had been assigned recently and could be attributed in OSM. This final step would mean: _Telus_ village is findable, people can be visited, mail can be delivered - mission completed [12]. After their session in Telus, the Filipino colleagues went straight to the next nearby GK village. GK officials showed vivid interest in the community mapping and _crowdsourcing_ approach, and we are pleased that we could initiate a meeting of local actors for a good cause. The uploaded material of [PERSON] and [PERSON] (Figure 9) forms a small step of progress concerning our work for GK. They included some samples of the tiny GK house footprints - each footprint is about 18 square meters. This detailed scale just could be reached by including the hand drawings of our MLA students and the (non-georeferenced) lot plan of the GK village (Figure 2); pure GPS mapping does not allow this specificity, and accuracy.Displayed at highest OSM zoom level, we realize that the house samples are nearly too small for such maps, but they are visible. The very small housing lots of GK slum upgrading projects would require a bigger zoom scale of the OSM map renderer (Mapnik). We would like to add more and more details to the OSM platform, but as yet closely located Points of Interest are hardly distinguishable and being labeled visibly. Not only free and open maps are getting more and more accurate, complete, and recent: e.g., _Baseco_ on _Google Maps_ has recently being updated [25]. Interestingly, this was also done by volunteers but using Google's _map maker_[26].