Archive for August, 2012

Land Concession Monitoring – From Space

With the advent of newer earth imaging satellites with daily overpass and capture possibilities, it is now even easier to monitor land use change. For example, we can examine the history of development of two Economic Land Concessions (ELCs) inside the Beng Per Wildlife Sanctuary. The concessions were granted in early 2010 and by March 2011, land clearing was well advanced.

Clearing of Land Concessions as of March 2011

If we examine the historical imagery, we can see the incremental clearing that has occurred over 1 year.

Historical Imagery: Mar-2010 -> Oct-2010 -> Mar -> 2011

 

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SPOT vs. SRTM DEM Comparison

Well I’ve finally manged to get my hands on some SPOT DEM data for Cambodia for a project in the Southern Cardamoms. Quite an improvement over the SRTM (90m) data.  The SPOT DEM has a resolution of 30m at the equator. From Astrium:

A SPOT DEM is a digital elevation model produced by automatic correlation of stereopairs acquired by the HRS instrument on SPOT 5.

Unlike a digital terrain model (DTM), which describes the distribution of terrain or “bare earth” heights, a DEM includes “first surface” elevations including vegetation and man-made structures.

SRTM DEM Detail. Scale approx 1:75,000

SPOT DEM Detail. Scale Approx. 1:75,000

For more info see http://www.astrium-geo.com/en/2790-elevation30-dem-spot-dem or please Contact Us

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Dispelling Myths about Garmin GPS Accuracy

The declaration of the accuracy by Garmin GPS receivers often leads to confusion. What does it mean if the receiver states an accuracy of 4 m? This readout refers to the so-called 50 % CEP (Circular Error Probable). This means that 50 % of all measurements are within a radius of 4 m. On the other hand, 50 % of all measured positions are outside of this radius. Furthermore, 95 % of all measured positions are within a circle of twice this radius and 98.9 % of all positions are within a circle of 2.55 the radius. In the given example, nearly all positions are within circle with a radius of 10 m. The determined position is in the worst case accurate to 10 m.

GPS Error Plot

 

Source : http://www.kowoma.de/en/gps/accuracy.htm

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Setting Up a Garmin GPS to Use Indian Datum

We often get asked about this question, so here are the full instructions on how this can be done:

Topographic maps in Cambodia use one of two datums; Indian 1954 and Indian 1960. By default, your Garmin GPS unit is set up to WGS84 datum, which is about 500m different to the Indian datums. If you want to display coordinates using one of the Indian datums, do the following;

  1.  In your unit, go to SETUP and then UNITS
  2. Go to  MAP DATUM and press the ENTER button
  3. Garmin supports the ‘Indian Thailand’ datum which is close to, but not exactly the same as the other INDIAN datums. The differences are:

INDIAN 54 vs. INDIAN Thailand: 2.2 meters

INDIAN 60 vs. INDIAN Thailand: 6.4 meters

Remember that standard GPS is generally accurate to about 10m, so these differences may be insignificant to you. If you want to be more precise, you can select the USER datum and enter the following parameters.

 

INDIAN 1954

INDIAN 1960

DX + 217 DX + 198
DY + 823 DY + 881
DZ + 299 DZ + 317
DA + 861 DA + 861
DF + 0.28361370 DF + 0.28361370

 

In Cambodia, the old 1:50,000 scale topographic maps are based on the INDIAN 1960 datum. The newer 1:100,000 scale maps from the Reconnaissance Survey Project (the “JICA Maps”) from 1998-2002 are based on the INDIAN 1954 Datum.

ផែនទីភូមិសាស្ដ្រមួយប្រើប្រាស់ក្នុងកម្ពុជានូវដាតូមពីរ ដូចជា ឥណ្ឌា ១៩៥៤ និង ឥណ្ឌា ១៩៦០ ។ ផែនទីចាស់មានមាត្រដ្ឋាន មូលដ្ឋានគ្រឹះខ្នាត ១:៥០,០០០ ដោយមានដាតូម ឥណ្ឌា ១៩៥៤ ។ ថ្មីៗ មាន ផែនទី ខ្នាត ១:១០០,០០០ ពី គំរោងអង្កេតការណ៍ ដែលជាគ្រឹះមានដាតូម ឥណ្ឌា ១៩៦០។

ដោយតម្រូវការរបស់ Garmin GPS ត្រូវមានខ្នាតចំពោះដាតូម WGS84 ជាមួយអំពីភាពខុស គ្នា ៥០០ម៉ែត្រ នៃដាតូមឥណ្ឌា ។ បើសិនលោកអ្នកចង់បង្ហាញនូវកូអរដោនេមួយ ដោយមានការប្រើ ប្រាស់ ដាតូមឥណ្ឌា ត្រូវអនុវត្ដតាមដូចខាងក្រោម ៖

1-    ក្នុងខ្នាតរបស់វា ត្រូវចូលទៅពាក្យ SETUP  និងបន្ទាប់មក UNITS

2-    ចូលទៅ MAP DATUM និង ចុចលើ ENTER ឧបករណ៍ ប៉ូតុង

3-    ជំនួយ Garmin វាមានដាតូម ឥណ្ឌាថៃឡង់ដិ៍ជាទៅចុងបញ្ចប់ ប៉ុន្ដែមិនសូវច្បាស់ លាស់  ដូចដាតូម ឥណ្ឌាដ៍ទៃទៀតទេ។ ភាពខុសគ្នាគឺ ៖

ឥណ្ឌា ៥៤ ជំនាន់ ខុសគ្នានឹង ឥណ្ឌាងៃឡង់ដិ៍ ៖ ២,២ ម៉ែត្រ

ឥណ្ឌា ៦០ ជំនាន់ ខុសគ្នានឹង ឥណ្ឌាងៃឡង់ដិ៍ ៖ ៦,៤ ម៉ែត្រ

ចូលចំណាំ ធម្មតា ស្ដង់ដារបស់ GPS ដែលមានតំលៃទៀងទាត់ប្រមាណ ១០ ម៉ែត្រ ដូនេះ ការខុសគ្នាប្រហែលមិនសំខាន់ ចំពោះអ្នក ។ បើសិនជាអ្នក ចង់បានភាពលំអិត ជាក់លាក់ លោកអ្នកអាចជ្រើសរើស ដាតូមប្រើប្រាស់ និង យល់ព្រមតាមកំណត់ដូច ខាង ក្រោម ៖

 

INDIAN 1954

INDIAN 1960

DX + 217 DX + 198
DY + 823 DY + 881
DZ + 299 DZ + 317
DA + 861 DA + 861
DF + 0.28361370 DF + 0.28361370

4-      ដើមី្បប្រៀបផ្ទឺម ផែនទីភូមិសាស្ដ្រ ទំរង់ទីតាំង តម្រូវឱ្យយដាក់ ’UTM UPS’ ក្នុង   Garmin GPS

 

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