Elevation data or information and its derivatives have been widely used in the management and analysis of spatial information (GIS) of an area. One that is often used is the Digital Elevation Model or DEM data which in English abbreviation is the Digital Elevation Model. Apart from DEM, there are also ‘similar’ data such as DTM and DSM.
There is no universal understanding of the terms Digital Elevation Model (DEM), Digital Terrain Model (DTM) and Digital Surface Model (DSM) in the scientific literature. In most cases, the term DSM represents the earth’s surface and includes all objects on it. In contrast to DSM, DTM represents the land surface without objects such as plants and buildings. DEM on the other hand is a database that represents the surface relief between known elevation points. By interpolating known elevation data from sources such as field surveys and photogrammetric data collection, a rectangular DEM network can be created. DEM only represents elevation information without further definition of the surface.
So far, there are many sources to get DEM data online and free of charge such as SRTM, Aster GDEM, ETopo, etc., but all of them are still at a relatively medium – small spatial resolution, nothing is detailed, especially for the context of the Indonesian region.
Digital Elevation Model National Aka DEMNAS
There is good news, that currently BIG or the Indonesian Geospatial Information Body has launched a new free DEM data service specifically for Indonesia which can be said to be better than the general free online DEM data. This data is known as the National DEM (DEMNAS) according to BIG it was built from several data sources including IFSAR data (5 m resolution), TERRASAR-X (5 m resolution) and ALOS PALSAR (11.25 m resolution), by adding the stereo-plotting Masspoint data. The spatial resolution of DEMNAS is 0.27-arcsecond, using the EGM2008 vertical datum.
Not only covering land, but the data also includes a model of seabed elevation or bathymetry. According to the BIG website for National Bathymetry, it was formed from the inversion of gravity anomaly data resulting from almetric data processing by adding sounding data conducted by BIG, NGDC, BODC, BPPT, LIPI, P3GL and other institutions with single and multibeam surveys. The spatial resolution of BATNAS data is 6 arc-second using the MSL datum. National Bathymetry gridded data from 90 to 150 E and from 20 S to 20 N. This bathymetric data has advantages in coastal areas and shallow waters using a survey from the Center for Marine and Coastal Environment (PKLP), BIG.
How to get DEMNAS?
#1 First, visit the site, create account and login
To be able to get the DEMNAS data, please access the DEMNAS BIG website or Ina Geoportal, including which you can get more detailed information about what and how the DEMNAS was created. Lintas Bumi has tried to download the DEM data for parts of Sorong Regency in West Papua.
The site is in Indonesian Language (Bahasa). On the site page on the top right you just click the Download menu. To be able to download it you must be registered, if not click the register menu and follow the steps in it until you get confirmation for account activation on your email, after that you just enter the email and password that has been emailed on the Login page.
#2 Second, select the desired region and download
After logging in the page will return to the Home page and you click Download again and navigate or zoom in (enlarge to) the desired area, until the following page appears.
Zoom in (you can also scroll the mouse) and scroll to the area you want, the DEMNAS data is divided into a grid with a length and width of approximately 27.85 km x 27.68 km or an area of 770.888 km2. To download it, please click on one of the grids, until the qlook DEMNAS image appears and click the DEMNAS code that appears below. The downloaded file format is GeoTIFF (Tif), please download the grids you need.
If you pay attention to the grid or index from the DEMNAS, it is actually the same as the RBI (Indonesia Topographic Maps) index with a scale of 1: 50,000, please prove it for yourself. So to make it easier to find DEMNAS data in locations that suit your needs, you can also start by checking using the RBI map index with a scale of 1: 50,000.
#3 Third, open the downloaded file
When Lintas Bumi tries to open the downloaded file in ArcMap, it turns out that the DEMNAS tif file is not georeferenced, so it is recommended that you first define projection (if using ArcGIS) on each downloaded DEMNAS file, the coordinates are the Geographic Coordinate System (GCS) WGS 1984. The results of the check shows the DEMNAS resolution is 8.33 meters which is better than the SRTM 1 Arc Sec or Aster GDEM (30 m) and SRTM (90 m).
The following is the DEMNAS of Bogor City and its surroundings opened at ArcScene. You can use the downloaded DEMNAS data for various purposes such as making contour maps, slopes, hillshades, and others. If needed you can also process several DEMNAS grids with mosaics to combine them into the DEMNAS area you want. Happy downloading.
Here is also a short video tutorial to download DEMNAS on Lintas Bumi TV channel (in Bahasa).