TUTORIAL ILWIS 3.6

Sebelum kita mengolah data di software ILWIS, mula-mula yang kita lakukan adalah memanggil/mengimport data yang akan diolah. Data yang akan kita olah adala data citra landsat, cara mengimportnya adalah sebagai berikut :

1. Setelah membuka software ILWIS 3.6, klik operation, pilik export/import, lalu klik import

2. Lalu buka data yang telah anda simpan sebelumnya, pada table directory klik data yang akan di import (misalkan yang pertama adalah band 1, lalu import formatnya diganti dengan use GDAL, output filename nya diisi dengan nama yang sesuai, samping kolom output filename anda klik, lalu tentukan letak data yang akan anda simpan (lebih baik anda simpan difolder data landsat yang sudah ada, tujuannya adalah agar data tidak berserakan), lakukan hal yang sama pada semua band.

3. Setelah semua band telah selesai diimport, lalu pada setiap band klik kanan, pilih statistic, lalu pilih histogram, lalu klik show (ini adalah tahapan sebelum membuat stretching)

4. Setelah itu klik kanan kembali, pilih image processing, lalu pilih stretch,

5. Pada kolom raster map pilih band yang akan distrech, lalu stretch methodnya klik linear stretching, lalu stretch fromnya klik min max, lalu isi dengan melihat pada histogram yang tadi anda buka sebelumnya, lalu domainnya menggunakan image, lalu output rastermapnya diisi, lalu klik show.

6. Setelah itu kita membuat maplist, yaitu dengan cara, klik file, pilih create, lalu klik maplist, pindahkan data yang akan disimpan dimaplist (misalkan yang akan disimpan dimaplist adalah data hasil strechingan tadi), lalu nama maplistnya diisi(missal : maplist_strech), lalu ok

7. Setelah itu kita membuat coordinat system, yaitu dengan cara : klik file, pilih creat, lalu klik coordinat system, lalu isi coordinat system namenya dengan wgs84, lalu ok

8. Pada kolom coordinat system boundary only, min x y dan max x y nya diisi dengan melihat data coordinat system dari band.

9. Selanjutnya kita membuat crooping, yaitu potongan/bagian yang akan kita buat supervise classification, dengan cara klik file, create, pilih georeference

10. Georeference name-nya diisi dengan nama crooping, coordinat systemnya menggunakan wgs84 yang telaah kita buat, lalu pixel size-nya 30, lalu min dan max xy-nya kita isi sesuai dengan crooping yang kita inginkan, lalu ok

Menutup” celah pada Citra Landsat SLC-Off

Tulisan ini saya buat karena saya kuatir saya akan segera lupa algoritma sederhana yang baru saja saya

tuliskan pada ERDAS Spatial Modeler untuk membantu “menutup” celah pada citra landsat SLC-Off. OK…

untuk memudahkan pembaca akan saya jelaskan sedikit lebih rinci dari awal.

Landsat adalah satelit pembawa sensor untuk pengambilan gambar bumi kita yang telah beroperasi sejak

1978 (Landsat 1… kalau tidak salah…, kalau penasaran dengan sejarah landsat silakan

buka http://landsat.usgs.gov/ selaku penguasa satelit ini). Saat ini, satelit yang masih beroperasi adalah

Landsat 5 dan Landsat 7. Nah sensor pengambil gambar (citra) yang terpasang pada satelit Landsat 7 ini

sejak akhir Mei 2003 mengalami kerusakan sehingga gambar yang dihasilkan selalu dihiasi dengan

“striping” yang berasal dari sebagian baris sensor yang tidak berfungsi lagi. Oleh USGS, citra-citra yang

diambil setelah tanggal ini disebut dengan citra yang bersifat “SLC-Off”. Contoh citra yang diambil dengan

kondisi ini adalah sebagai berikut

Citra Mei 2008 (SLC-Off)

Untuk”mengoreksi” celah yang timbul akibat kerusakan sensor pada satelit landsat 7, kita bisa

menggunakan citra pada path/row yang sama yang diambil pada tanggal/tahun yang berbeda. Untuk contoh

ini saya gunakan citra yang diambil pada Mei 2003. Secara tata waktu mungkin citra ini “terlalu tua”, namun

citra ini saja lah yang mengandung informasi yang dapat digunakan untuk “menambal” garis-garis kosong

pada citra Mei 2008 tadi. Perlu diingat bahwa penyedia data citra biasanya memberikan nilai (digital

number/DN) = 0 untuk pixel-pixel yang mengalami “striping” alias tidak berisi data. Nah, nilai-nilai 0 inilah

yang nantinya akan digantikan oleh nilai pixel yang berasal dari citra “penambal” yang satu lagi.

Untuk citra yang akan digunakan untuk “menambal” ditampilkan sebagai berikut:

Citra Mei 2003

Perlu diingat juga sebelum melakukan operasi ini, kita harus memastikan posisi geometris kedua citra

sudah sama persis. Lakukan koreksi geometrik menggunakan titik ikat yang diukur di lapangan (mis: dengan

menggunakan GPS saat survey) pada citra yang pertama, kemudian sesuaikan/koreksi posisi geometrik citra

yang kedua berdasarkan hasil koreksi geometrik citra yang pertama sehingga diperoleh posisi geografis

yang tepat sama untuk kedua citra. Setelah syarat ini dipenuhi, kita bisa mulai mencoba “menambal” citra

yang bergaris ini.

Saya menggunakan ERDAS Imagine v9.1, tapi saya yakin versi yang lain tidak memiliki perbedaan dalam hal

bahasa/perintah yang diimplementasikan dalamspatial modeler nya. Gambar berikut memuat algoritma

sederhana yang digunakan dalam ERDAS Model Maker untuk melakukan proses ini. Gambar yang dimuat

memang hanya menampilkan Function Definition pada model yang dibuat pada ERDAS Model Maker.

Spatial Modeler: Conditional

Saat model dieksekusi, kita dapat memasukkan citra yang “bergaris” sebagai input pertama

($n2_PROMPT_USER) sementara citra yang utuh sebagai input yang kedua ($n3_PROMPT_USER). Tentukan

nama dan lokasi citra sebagai output, dan hasilnya akan tampak seperti berikut:

Citra Landsat Mei 2008 (Hasil)

Tentunya, pemilihan citra pengganti, tingkat akurasi koreksi geometrik dan kondisi lapangan sangat

mempengaruhi ketepatan informasi yang dapat digali dari proses ini. Saya juga TIDAK

MEREKOMENDASIKAN penggunaan teknik ini untuk pengolahan citra satelit yang melibatkan secara

langsung digital number/DN ataupun nilai spektral asli dari citra satelit. Praktek ini dianjurkan hanya

untuk identifikasi tutupan lahan, itupun harus dilihat dengan cermat kondisi di lapangannya melalui

survey. Akhir kata, semoga catatan kecil ini berguna bagi pengguna ERDAS Imagine lainnya.

1 8 F E B R U A R Y 2 0 1 1

Free SRTM 90m Data for the entire world !

Download SRTM 250m or 90m Digital Elevation Data (Version

4.1)The SRTM Data now available from CSI-CGIAR SRTM Data

portal. This is Free, and all the data has been upgraded to version 4.1.

This latest version represents a significant improvement from previous

versions, using new interpolation algorithms and better auxiliary

DEMs. 

For quick and easy data downloads, this portal has a nice user-friendly web interface which

provides users three flexible options to choose, in order to selecting multiple tiles for

downloading.

1. Multiple tile selection by clicking on tiles

2. Drag the mouse-point on top of the world map and mark & select the tiles

3. or, enter lat-long input coordinates.

In addition, you can switch between their different hosting servers, to select either HTTP

or FTP downloads.

Just open the below image and see how easy it is !

The CGIAR-CSI GeoPortal is able to provide SRTM 90m and 250m Digital Elevation Data for

the entire world. The SRTM digital elevation data, produced by NASA originally, is a major

breakthrough in digital mapping of the world, and provides a major advance in the

accessibility of high quality elevation data for large portions of the tropics and other areas of

the developing world. The SRTM digital elevation data provided on this site has been

processed to fill data voids, and to facilitate it's ease of use by a wide group of potential

users. This data is provided in an effort to promote the use of geospatial science and

applications for sustainable development and resource conservation in the developing world.

Digital elevation models (DEM) for the entire globe, covering all of the countries of the world,

are available for download on this site. 

The SRTM 90m DEM's have a resolution of 90m and 250m at the equator, and are provided

in mosaiced 5 deg x 5 deg tiles for easy download and use. All are produced from a

seamless dataset to allow easy mosaicing. These are available in both ArcInfo ASCII and

GeoTiff format to facilitate their ease of use in a variety of image processing and GIS

applications. Data can be downloaded using a browser or accessed directly from the ftp site

For more information and direct FREE data downloads,

visit:http://srtm.csi.cgiar.org/

OR If you want to go DIRECTLY to Data Download page: CLICK HERE !

       SRTM 90m Digital Elevation Data

Resampled SRTM data to 250m resolutions for the entire globe are available

https://hc.box.net/shared/1yidaheouv (Password: ThanksCSI!)

UPDATE - VERSION 4: THE SRTM DATA NOW AVAILABLE FROM THIS SITE HAS BEEN UPGRADED TO VERSION 4. THIS LATEST VERSION REPRESENTS A SIGNIFICANT IMPROVEMENT FROM PREVIOUS VERSIONS, USING NEW INTERPOLATION ALGORITHMS AND BETTER AUXILIARY DEMs.  WE ARE CONFIDENT THIS IS NOW THE HIGHEST QUALITY SRTM DATASET AVAILABLE

The CGIAR-CSI GeoPortal is able to provide SRTM 90m Digital Elevation Data for the entire world. The SRTM digital elevation data, produced by NASA originally, is a major breakthrough in digital mapping of the world, and provides a major advance in the accessibility of high quality elevation data for large portions of the tropics and other areas of the developing world. The SRTM digital elevation data provided on this site has been processed to fill data voids, and to facilitate it's ease of use by a wide group of potential users. This data is provided in an effort to promote the use of geospatial science and applications for sustainable development and resource conservation in the developing world. Digital elevation models (DEM) for the entire globe, covering all of the countries of the world, are available for download on this site. The SRTM 90m DEM's have a resolution of 90m at the equator, and are provided in mosaiced 5 deg x 5 deg tiles for easy download and use. All are produced from a seamless dataset to allow easy mosaicing. These are available in both ArcInfo ASCII and GeoTiff format to facilitate their ease of use in a variety of image processing and GIS applications. Data can be downloaded using a browser or accessed directly from the ftp site. If you find this digital elevation data useful, please let us know at csi@cgiar.org

The NASA Shuttle Radar Topographic Mission (SRTM) has provided digital elevation data (DEMs) for over 80% of the globe. This data is currently distributed free of charge by USGS and is available for download from the National Map Seamless Data Distribution System, or the USGS ftp site. The SRTM data is available as 3 arc second (approx. 90m resolution) DEMs. A 1 arc second data product was also produced, but is not available for all countries. The vertical error of the DEM's is reported to be less than 16m. The data currently being distributed by NASA/USGS (finished product) contains "no-data" holes where water or heavy shadow prevented the quantification of elevation. These are generally small holes, which nevertheless render the data less useful, especially in fields of hydrological modeling.

Dr. Andy Jarvis and Edward Guevara of the CIAT Agroecosystems Resilience project, Dr. Hannes Isaak Reuter (JRC-IES-LMNH) and Dr. Andy Nelson (JRC-IES-GEM) have further processed the original DEMs to fill in these no-data voids. This involved the production of vector contours and points, and the re-interpolation of these derived contours back into a raster DEM. These interpolated DEM values are then used to fill in the original no-data holes within the SRTM data. These processes were implemented using Arc/Info and an AML script. The DEM files have been mosaiced into a seamless near-global coverage (up to 60 degrees north and south), and are available for download as 5 degree x 5 degree tiles, in geographic coordinate system - WGS84 datum. These files are available for download in both Arc-Info ASCII format, and as GeoTiff, for easy use in most GIS and Remote Sensing software appications. In addition, a binary Data Mask file is available for download, allowing users to identify the areas within each DEM which has been interpolated.

MIRROR DOWNLOAD SITE: Dr. Mark Mulligan (King’s College London) mirrors the data, and has created a Google Earth Interface for browsing and downloading SRTM tiles. it alsoprovides smaller (1 by 1 degree) tiles for users who have difficultywith the 5x5 degree tiles as well as 2D and 3D visualisation of the data.

At one time, Virtual Reality Markup Language (VRML) was going to be the next big thing on the Web; fully-interactive 3D environments were going to change the way we interacted with the Internet, and our computers. Didn’t quite work out that way – most people were on dial-up back then, which was too slow to make VRML practical. Even after broadband became more prevalent, it turned out that most people didn’t really want to interact with their computers that way. For displaying maps in an interactive 3D environment, Google Earth, World Wind, and other similar programs have pretty much supplanted VRML. But there may still be a place for it in some applications: you can use higher-resolution terrain than what’s currently available in Google Earth/WorldWind along with your own image overlays, the computing requirements are reduced, and the filesize for VRML worlds is usually pretty small. And you can get both stand-alone VRML viewers and browser plug-ins for multiple platforms for free.

3DEM makes creating a VRML world easy. From   yesterday’s post , if you’re in a “Foreground” terrain view:

Select choose File=> Save VRML World; a file save dialog will open asking where you want to save the files. Next, you’ll be asked to specify some additional parameters for your VRML world:

       SRTM 90m Digital Elevation Data

Resampled SRTM data to 250m resolutions for the entire globe are available

https://hc.box.net/shared/1yidaheouv (Password: ThanksCSI!)

UPDATE - VERSION 4: THE SRTM DATA NOW AVAILABLE FROM THIS SITE HAS BEEN UPGRADED TO VERSION 4. THIS LATEST VERSION REPRESENTS A SIGNIFICANT IMPROVEMENT FROM PREVIOUS VERSIONS, USING NEW INTERPOLATION ALGORITHMS AND BETTER AUXILIARY DEMs.  WE ARE CONFIDENT THIS IS NOW THE HIGHEST QUALITY SRTM DATASET AVAILABLE

The CGIAR-CSI GeoPortal is able to provide SRTM 90m Digital Elevation Data for the entire world. The SRTM digital elevation data, produced by NASA originally, is a major breakthrough in digital mapping of the world, and provides a major advance in the accessibility of high quality elevation data for large portions of the tropics and other areas of the developing world. The SRTM digital elevation data provided on this site has been processed to fill data voids, and to facilitate it's ease of use by a wide group of potential users. This data is provided in an effort to promote the use of geospatial science and applications for sustainable development and resource conservation in the developing world. Digital elevation models (DEM) for the entire globe, covering all of the countries of the world, are available for download on this site. The SRTM 90m DEM's have a resolution of 90m at the equator, and are provided in mosaiced 5 deg x 5 deg tiles for easy download and use. All are produced from a seamless dataset to allow easy mosaicing. These are available in both ArcInfo ASCII and GeoTiff format to facilitate their ease of use in a variety of image processing and GIS applications. Data can be downloaded using a browser or accessed directly from the ftp site. If you find this digital elevation data useful, please let us know at csi@cgiar.org

The NASA Shuttle Radar Topographic Mission (SRTM) has provided digital elevation data (DEMs) for over 80% of the globe. This data is currently distributed free of charge by USGS and is available for download from the National Map Seamless Data Distribution System, or the USGS ftp site. The SRTM data is available as 3 arc second (approx. 90m resolution) DEMs. A 1 arc second data product was also produced, but is not available for all countries. The vertical error of the DEM's is reported to be less than 16m. The data currently being distributed by NASA/USGS (finished product) contains "no-data" holes where water or heavy shadow prevented the quantification of elevation. These are generally small holes, which nevertheless render the data less useful, especially in fields of hydrological modeling.

Dr. Andy Jarvis and Edward Guevara of the CIAT Agroecosystems Resilience project, Dr. Hannes Isaak Reuter (JRC-IES-LMNH) and Dr. Andy Nelson (JRC-IES-GEM) have further processed the original DEMs to fill in these no-data voids. This involved the production of vector contours and points, and the re-interpolation of these derived contours back into a raster DEM. These interpolated DEM values are then used to fill in the original no-data holes within the SRTM data. These processes were implemented using Arc/Info and an AML script. The DEM files have been mosaiced into a seamless near-global coverage (up to 60 degrees north and south), and are available for download as 5 degree x 5 degree tiles, in geographic coordinate system - WGS84 datum. These files are available for download in both Arc-Info ASCII format, and as GeoTiff, for easy use in most GIS and Remote Sensing software appications. In addition, a binary Data Mask file is available for download, allowing users to identify the areas within each DEM which has been interpolated.

MIRROR DOWNLOAD SITE: Dr. Mark Mulligan (King’s College London) mirrors the data, and has created a Google Earth Interface for browsing and downloading SRTM tiles. it alsoprovides smaller (1 by 1 degree) tiles for users who have difficultywith the 5x5 degree tiles as well as 2D and 3D visualisation of the data.

Surface Smoothness and World Dimension set the effective resolution for your VRML world. Flat is less detailed than Smooth, and creates a smaller file size, but given the speed of modern computers, there’s little reason not to set Surface Smoothness and World Dimension to their maximum. Click OK, and two files will be created: an *.wrl file, which contains the terrain data, and a *.jpg file that contains the image data that overlays the terrain. You’ll need to keep both files together in the same folder when viewing the VRML world

To view the result, you’ll need a VRML viewer. Here’s a screenshot from GLView:

Using the controls in this program, I can walk or fly around the terrain, zoom in and out, change my viewing angle and altitude, take snapshots, and so on.

GLView is the best VRML viewer I’ve seen, but unfortunately it’s no longer available for public distribution. But there’s a goodVRML browser plug-in called Cortona available for free from Parallel Graphics. There’s also a stand-alone VRML viewer available from Octaga, but I haven’t actually tried that one myself.