5. Conversion of data > 5.3 Raster maps
5.3.7 Download Google Earth Image
Level of expertise required for this Chapter: Advamced; general GIS training
While it appears, that Google Earth has raster images, which can be used in LUPMIS and therefore no need to procure data, this is not feasible for a variety of reasons as explained below. It is therefore NOT recommended to use Google images for professional work.
1. Google images are the intellectual property of Google or the image provider. Images can only be downloaded and used, when a user agreement has been settled. (As far as I know, only possible against payment).
2. Google does not give any guarantee on accuracy. If the images satisfy quality expectations on 3 or 5 or 10 m: Who knows?
3. Google image resolution are not always good enough for spatial planning. Often, major towns show a good resolution (often less than a meter), but not so outside urban areas.
4. Google Earth does not display images in linear UTM projection. The error due to different projection can only be neglected on extremely large scales.
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Only for technical gurus, not for production purposes, the earlier description of the download process is kept in this Chapter.
Legal Note: Documentation below explains the technical process. It does not authorize the use of these images. Legal conditions have to be checked on a case-by-case base; they can change on short notice. Downloaded images are only for orientation and non-commercial use.
1) Define the coordinates in lat / long decimal degrees (for example: Left Longitude: 0.04 > Right Longitude: 0.05 > Top Latitude: 5.76 > Bottom Latitude: 5.75). Don’t have more than 0.01 deg in latitudinal and longitudinal direction (approximately 1 km) to avoid geographical distortion. Western longitude values are entered as negative numbers.
2) Load the software Google Satellite Maps Downloader. (If not already installed, procure it on-line from the internet or contact TCPD-HQ / LUPMIS for advice)
3) Google Satellite Maps Downloader: Project name: LUPMIS > Longitude and latitude: As defined in step 1 above > Zoom: 18 > Threads count: 4 > Specify path to save > Download
Note: Zoom level 18 has a resolution (pixel size) of 60 cm, very often ‘QuickBird’ data.
4) View the downloaded images: Tools > Satellite images viewer
5) Combine the images to one: Tools > Combine images > Select GMID file > Combine. Downloading might take some time….
6) In Paint Shop Pro: Convert BMP format (16-bit) to JPG format (File > Open > Select folder and filename, under BMP format > Open > File > Save As > Type: JPG > Save)
If necessary, enhance colours / contrast.
7) View log file and read the lat / long coordinates of the entire image (in ..._log.txt): Tools > View log now
8) Load the conversion program GeoTrans95: Convert two corners to UTM.
See Chapter 5.1.3, with Input: Geodetic, Output: UTM. Be aware, that you might have to enforce the UTM zone (30 or 31).
9) In Map Maker: Georeference the image: Main menu > Utilities > Bitmap utilities > Calibrate scans > Two known points and use the NW and SE corner to calibrate (see Chapter 5.3.3).
10) Display the map in Map Maker and assess the horizontal accuracy against GPS data.
11) If necessary, minor shifts can be made in Map Maker (see Chapter 5.3.6).
12) Repeat these steps 1-11, till you have covered your entire area of interest.
13) In Map Maker: Mosaic all individual images to one large image (Main menu > Utilities > Bitmap > Bitmap utilities > Mosaic and clip, see Chapter 5.3.1-D).
Do not exceed maximum recommended size (6x3 km at a resolution of 80-100 cm, see Chapter 5.3.1-D).