When we create GIS project to natural resources study we need information about situation of surface.  It mains, the information have the relationship with surface or be build from third-dimensional function. This fact, only use for continuous data like as rain, elevation, slope etc.  So, surface can be defining as presentation continuous spatial phenomena. 

            As third-dimensional function, surface can be presentation into xyz coordinate, because that if we will build the project based on surface we must to sure about our entity of GIS data or shape contains the height attribute as X-axis.  Many continuous data can be presentation using XYZ coordinate.  For example, distribution of temperature on the land surfaces area, contour, rain or other.  X and Y-axis may present about point or where the location and Z is subject value.  On the real world X and Y may the coordinate value as function from latitude and longitude and Z is continuous data value.  Figure 1 as shown coordinate system used for surface.

               Using surface can create differential information as surface function.  From surface data can create some terminology like as Digital Elevation Model (DEM), Digital Terrain Model (DTM), Lattice or Grid and Triangulation Irregularly Network (TIN).  Especially, for DEM and DTM we should have the information about height or elevation as attribute in the database of shape or coverage as Z value.  The value of elevation can be presented as a grid of points distributed through the area or we often called as a raster form organization.  DEM and DTM often use to topography analysis as geology, agriculture subject or other. 

                Actually, for Z value of surface not only elevation, but also presented another continuous data.  Therefore, we must to consider how to create the surface using some information or continuous data.

Figure 1.  The coordinate system may use into surface process

A.  Creating Surface 

Creating surface are important stage if we develop the GIS project using surface information.  It is initial stage. The data which often use to make initial surface is elevation, because it data is reference data and can use to combine with the other continuous data so that will make easier to next analysis. So that to create a surface will focus to elevation data.  The methodology will be used to capture and store elevation can be grouped into several basic approaches as Regular Grids or Lattices, Contour and TIN. About the data that as source to generated digital elevation model or surface can from many sources like as contour map, aerial photo and satellite image with high resolution.  For aerial photo and image data, must to analysis for getting the stereo data.

1.  Grid / Lattices

Grid definitions

Grid is the one of all data providing for digital terrain or elevation model, otherwise also another continuous data.  The data have grid format.  According Gibaeu (2000), a grid defines geographic space as a matrix of identically sized square cells.  Each of cells holds a numeric value that measures geographic attribute for that unit of space.  For example, elevation data value will store for each set of regularly spaced ground positions, so that each data point represent the elevation of the grid cell in which each of location. About it often called Digital Elevation Model (DEMs). 

 Grid Processing 

                If we have the data contained z value, we can create grid data.  Value each of cell will be interpolated to determine the unknown data between points that have z value.  In this case, interpolation can define as the procedures of predicting or calculating unknown values using the known data values at neighborhood locations.  To interpolate, some software have been established algorithm.  Figure 3 as show how and where the cell will be give the value as interpolation result.  The result of interpolation will be influencing some factors and they also determine the quality of interpolation.  They are accuracy of data, number of data (not all many data will give the good quality interpolation, it is depend on distribution of data), distribution of known points and how well the mathematically function that used to interpolation, because in the some software we can select it function.  So that considering from those factor, we must to understanding about data characteristic and objective of interpolation so that we can determine mathematically function will be used. 

2.  Triangulation Irregularly Networks (TIN)

TIN definitions

                Some times, when we create GIS project using surface we only have discrete data.  Actually from the theory we cannot direct create surface.  TIN data model give solving problem it.  Using TIN, makes possible for set data points to be represented as a continuous surface.  It is replication of much of the functionality of grids in a vector context.  As simply, TIN is geometric term that a set vertices or point that are connected by edges that from triangle dimension as shown in Figure 4.  TIN model will make height between points or node can be interpolated thus allowing the definition of a continuous data. 

            TIN have two element for data structure; first, a set of input points with X,Y, Z values and second is a series of edges connecting these point to form triangle.  Data input each of point will be changes into the node of a triangle and the output is a continuous face surface of triangles (see Fig,2.3).  Connecting between nodes neighborhood will be closed area and became triangle polygon.  Refer from explanation how TIN processing we could say that a TIN has a more complex data structure data structure than grid.  Because it must stored 3D coordinate for each point or nodes and maintain appropriate topological relationship between adjacent triangles.

TIN processing

           Like as on above explanation, we have been known that TIN processing is connecting the data points.  It process called tessellate, it is filling plane with repetitions of figures in away that this place not have information.  Triangulation generated based on Delaunay triangulation mathematical technique.  It is can be define that a circle drawn from the three points of any triangle will contain no other input point.  It is give guaranteed that the polygon of triangle no overlap with the other polygons, it is will be generated one by one triangle each of point with neighborhood point as shown in Figure 2.4.  The output TIN processing can make possible for interpolation zvalue for any location a TIN surface using the XYZ valueof the bounding triangle points.  We also fill additional information with calculate for each face such as slope, aspect and surface area.

 Figure 2.4.  Delaunay approach