With the new Background Map tool of DigiTerra Explorer it is possible prepare individual base maps from online sources and use them during data collection. This can be done without Internet connection in the field!
Navigation with free high-resolution aerial imagery and maps
During field data collection, a satellite image, an aerial photograph or even a topographic map improves significantly the quality of mobile mapping. All these data are accessible but often with difficulties and high costs. However, when using smart phones during online navigation or browsing websites, several free aerial maps are available from online sources. The question arises: if the online version is free, why can't we take them with us to the field? The data would still be free, but we should use a mobile phone with GPS in one hand for displaying maps, and a PDA for data capturing in the other hand. Surely, this would be expensive and extremely inconvenient.
GIS data capture with expensive aerial photos?
- Why don't we apply both together?
- Why can't we export online backgound maps into DigiTerra Explorer?
The answer - which might be less known - is quite simple: GIS data providers are protecting their data and apply strict licencing rules combined with extremely high costs for using these for business purposes. The reason behind is their intention to make money on advertising in connection with Location Based Services: places that are willing to pay for LBS, will be highlighted and recommended on the map based on the user’s current position: a restaurant, a boutique or a club. It is not a coincidence that maps have better quality in the urban area. Outside of cities or on the sea the same maps are poorer and still not free.
Free background map for GPS measurements with DigiTerra Explorer 6
Good news for those who would hurry to the providers to buy data! DigiTerra has developed a solution to import a map data package from any source and make it accessible anywhere in the world in any projection in the background. From our perspective this 'only' means supporting file formats, transforming projections and showing the layer. Compared with online background maps, this method needs some preparation in the office, but in return no data transfer costs arise and the solution is available in each DigiTerra Explorer version free of charge!
Can it be this easy? Do I only have to click on a button and the aerial photo appears in the backgound online? Yes, it's possible, but it has costs which is what most of our customers can't afford. In addition, there's often no GPRS signal in the field, or if there is, the connection is slow. For this reason we have to use a slightly more complicated, two-step method.
Let’s see how it works…
The key: Mobile atlas creator (MOBAC)
There is a free, open source, JAVA based smart image tile downloading program that runs on all computers and operating systems, called Mobile Atlas Creator (MOBAC)), formerly known as Trek Buddy Atlas Creator. The application is very easy to handle, it supports a number of data sources and not only the well-known ones. At the same time, you can select numerous output formats. Nevertheless, the downloading of maps should be done on one's own responsibility because the licences of the map providers are different. The OpenStreetMaps are certainly free of charge.
Download MOBAC v 1.8 here!
How do I prepare the background map for taking it to the field?
Step 1 - download tiles
- Select a data source: street map, topographic map or aerial photo. The difference between different data can be seen at a glance, you can change between them with one click.
- Select resolution and necessary details for the photo or map. It is recommended to limit downloaded image tiles to a few thousand.
- 3. Select the area for which you need the offline map – practically the area where you will be measuring.
- Create an ‘atlas’ from this selection and save it in Osmdroid ZIP format.
- The ZIP file shouldn't be larger than 50-100 MB to ensure a good performance on a PDA.
Step 2- use background maps in DigiTerra Explorer
After the preparation, start DigiTerra Explorer, go to the background map menu an import the relevant zip file. If you have download data from different sources, you have the option to select the most appropriate one. Now you can start measurement and data collection with a street map, a topographic map or an aerial image in the background. Every offline background map is PROJECTION-INDEPENDENT, so they can be used automatically in any projection. Also the appropriate resolution is adjusted automatically by DigiTerra Explorer. After finishing data collection, you have also the option to use KML export, as a result you can publish your measurements with an ortophoto or satellite image background.
Considerations – data quality issues
There is no doubt: free online sources (especially outside of urban area) offer heterogeneous data quality, poorer resolution and lower accuracy then expensive providers. Often the capture time of the image can't be assessed, but usually they are less up-to-date. On the other hand, their indisputable advantage is that they are free of charge and available for the whole area of the world. Here are some examples to consider regarding data quality issues.
When comparing Google Earth and Microsoft Virtual Earth, the difference is insignificant, only tones differ:
If you add an official real estate map, you can see that Microsoft (on the right) has a better accuracy than Google, but there are still differences:
You can also notice big differences in image quality: Microsoft’ image (down) is more homogenous and constant in this case.
Resolution outside of cities is even worse, a tourist map reveals sometimes more than remote sensed photos. Here for example, at level 18 Microsoft (down) does not provide sufficient information:
