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Topographic map.

Producing Digital Terrain Model from topographic maps is an old fashion procedure. Although it is an old

fashion, it still need a computer to help in the conversion process to make it less tedious as the manual

process needs a lot of time. Today, topographic maps is still in use, for example, “the shaded relief maps

appearing on the front pages of our national publications were not produced from satellite data because

that data is classified. Instead, they were derived from the most primitive but reliable source of DEM

data that we have: paper topographic maps” 1.

There are several process involves in creating Digital Terrain Model. The traditional method for

vectorization of contour line involves mainly the following steps:

1) Scanning paper topographic maps using high resolution scanner.

2) Registration of one or more maps with reference to the nearest datum.

3) Mosalcing or stitching various topographic maps.

4) Vectorization of various contour lines manually using line tracing by rubber band method.

5) Feeding depth information for each contour line.

6) Generating digital elevation models (DEM) for 3D surface reconstruction.

Uses of computer and digital topographic maps have made the task simpler:

1) Registration of topographic map.

2) Filtering for enhancing map.

3) Colour segmentation for extracting contour lines.

4) Thinning and pruning the binary images.

5) Raster to vector conversion.

Producing DTM from topographic map will followed by several problems. Existing topographic

maps include much noise due to colour variation, grind lines, letters and symbols for identifying contour

lines. This noise can be eliminated, semi- automatically or interactively by computer. Forget about the
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semi-automatically which is so tiring and tedious, the computerize way of were used in two ways which

is created in Japan: a) Contour Following Method for raster output data, b) Vectorization Method for

vector output data2.

Then the problem of obtaining the suitable colour raster image of a topographic map. A raster

topo sometimes will look pretty in the screen but will rapidly turn to unusable mush upon multiple

separation scans1. It needs a high resolution image to run this process.

Thirdly, the separation of contour lines from other unwanted raster data. The process needs a

filtering process by pixel RGB ( red-green-blue) value. In this process, the unwanted raster data will be

remove1. Then followed by the connection as many broken contour lines and erase as many single lines

which cannot be connected with other lines as possible.

The accuracy of DTM produced from topographic map is dependent. Its depends on the

accuracy of the topographic map itself and today there are many software involves in producing DTM

from topographic maps were introduced, therefore the task of conversion will be easy with lack of

problems
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Satellite image.

Satellite imagery basically use the same concept as used in aerial photogrammetry which utilizes stereo

pairs of satellite imagery that cover approximately the same area but in smaller scale mapping and DTM

production. Satellite imagery that used in photogrammetry provides a vertical accuracy of 1-10 meters.

Digital images can be geometrically processed to achieve high accuracy which would be

comparable to cartographic maps of similar scale. They can be geometrically corrected and geocoded

using accurate methodology and accurate DTMs to represent terrain mapping units.

However, satellite imagery have geometrical errors and it have more errors compared to aerial

photos. Moreover, there are two additional common errors, they are instrumental errors and

environmental errors. Instrumental errors include errors caused by imperfection in instrument

construction, or lack of instrument adjustment or calibration prior to its use in data collection. While the

environmental errors are caused by variations in atmospheric conditions, variations in platform altitude,

velocity and attitude, satellite ephemerides, earth rotation, earth curvature, variations of sensors timing

and other minor sensors scan non-linearities. Fortunately, all the above errors listed have their own

corrections but the environmental errors have the least corrections compared to others.

The achievable accuracy of a satellite imagery is dependent on the image scale, pixel ground

resolution, orthoimage scale and the mapping accuracy specifications which stated in FGDC 1998.

There are researches that continue until now to improves and to tackle several problems that

are faced to widening the use of satellite imagery. For example, satellite imagery is often unavailable

and very expensive to acquire. Then interpolation technique were used which is fast and cheap, but it

have their own inherent difficulties and problems, especially in rough terrain. Therefore, a new research

of shape from shading technique is run, which only deals with a single high resolution satellite imageries

which almost can be obtain worldwide. This methods will define the grid heights of an area although the

reconstruction of objects from monocular image is very difficult and in some cases, it is impossible. The
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research is continue because if it is done and approve to be use, it will bring so much benefits and

advantages.

Real Time Kinematic GPS.

Real Time Kinematic GPS is one well known instrument which is great in obtaining X, Y, and Z

coordinates and directly shows the result on the exact moment comparing with other GPS coordinate

obtaining techniques. RTK GPS offers an alternative for surveyors to run a survey job under various kind

of weather and conditions of open space.

RTK GPS survey requires two control points or benchmarks, one benchmark as its reference

station and another one as its initial points or so called rover. The coordinates of the base station is used

as the initial coordinates for the derivation of rover’s RTK positions throughout the survey area. RTK GPS

is set up on a known coordinates point while the rover moves and pick up necessary topography points

which is then the links of measurement obtain at the reference receiver to the roving allows the real

time calculation of the rovers coordinates.

The accuracy obtain will be in centimeter level which is good enough in producing high accuracy

DTM. The DTM processing will be done in various software such as ArcGIS, TIN, and CDS. There will be a

network of triangles showing the terrain in digital.

Although using RTK GPS technique is the easiest, cheapest, and fastest way with high accuracy,

it cannot run from several errors and weaknesses. In the field, RTK GPS will face problems when it comes

to an mass urban area or forest. There will be so many obstructions to get a clear signal from the

satellites that passed. Moreover, it needs several man to run and collecting the data which also counted

as cost. Lastly, it can only covers a few kilometer squares as the baseline not recommended to be

further than 10km.

References

1) http://www.terrainmap.com/rml9.html

2) Automated Height Information Acquisition From Topographic Map;

Sukit Viseshsin & Shunji Murai, University Of Tokyo,

3) Contour Line Tracing Algorithm For Digital Topographic Maps;

Ratika, Shikhar, Ruchika, Mohan & M.K. Ghose, Department of CSE, SMIT, Rangpo, Sikkin, INDIA.

4)Estimating The Quality Of Digital Terrain Model Generated From RTK GPS Data On Terrain With

Various Sky Of Views; Dedi, Cahyono & Abd. Nasir, Universiti Teknologi PETRONAS, MALAYSIA.

5) Digital Terrain Modeling By Real Time Kinematic GPS;

Dedi, Bambang, & Abdul Nasir, Department Of Geodetic Engineering, Universitas Gadjah Mada.

6) Densification Of Digital Terrain Elevations Using Shape From Shading With Single Satellite Imagery;

Mohammad A. Rajabi & J.A. Rod Blais, Department OF Geomatics Engineering, University Of Calgary.

7) Geometrically Corrected Digital Images For Resource Mapping And Precision Farming;

Albert K. Chong & Chris Pearson, Department Of Surveying, University Of Otago.