Vector Spatial Layers and End Product - Essay Example

Vector Spatial Layers and End Product In order to create the final product two vector spatial layers are required, with the buildings in the map represented by a polygon feature class and the streets by a polyline feature. In addition, tables are also used to reduce the redundancy and duplication of the data, containing the land use types, with a number associated with each type. What attributes should be associated with your features and how should the attributes be defined (Type, Length, Allow null values, set default values, etc)? Explain the need for each field. Due to the fact that data may not be available or missing, all fields in the two feature classes will accept a NULL value. Both feature classes have no default value because the input data is various. Field names and their data type are shown below: # for the street feature class: Field Data Type Length OBJEC TID Object ID - SHAPE Geometry - Street_name Text 20 Street_Type Text 5 R_from Long integer - R_to Long integer - L_from Long integer - R_to Long integer - Street_year Short integer - → Street_Name: this field will hold the street name → Street_Type: this field will hold the street type (Rd, Ave ...etc.). → L_from, R_from, L_to and R_to fields: these fields are necessary for the geo-coding operation later, which will link to the numbers for the right and left sides of the street. → Street_Year : year of the tile map, to avoid mixing this feature class with other feature classes from other periods. # For the Building feature class: Field Data Type Length OBJECTID Object ID - SHAPE Geometry - Land_use Short integer - Specific_building_usage Text 50 Number_of_stories Short integer - Building_year Short integer - Address Text 50 → Number_of_stories: contains the number of the stories per building which will help to carry out the city visualisation. → Land_use: this will state the type of the buildings (Commercial, Residential or Public). For this field, short integer data type was associated with it in order to link the field with a table that holds the IDs for each type. → specific_land_use : lists the actual use of the buildings (School, or warehouse). → Address field: this will contain the address to street level after the geo-coding operation. Year: again, the year of the tile map, to avoid mixing this feature class with other feature classes from other periods. How will you associate a designation of the time period, 1920, with the feature classes you create? Be mindful of the integrity of your data versus considerations of data-redundancy. Justify your decision. The only way to associate the time period is to add a field in both feature classes to feature the year of the building and the street, which will also help to avoid mixing different data, because the project will cover 10 year intervals and there will be large number of feature classes. Another way is to add the time period to the title of the shape file, for example (streets_1920 or Buildings_1920) What do you need to do in order for the image file of the orthophoto to be properly prepared for the project? (The DOQQ.jpg file) In order to use the orthophoto file, the spatial reference should be defined to (UTM Zone 17 North, NAD83). This step could be done using ArcCatalog by entering the properties of the layer and then selecting the spatial reference from the projection folder. Another way to define the projection is by using the projection tool in Arc Tool box. I alluded above to the fact that you will georeference the Sanborn map image files based on the orthophoto. Why not use the DRG (the o38078a4.tif file) instead of the orthophoto? The reason for not using the DRG file instead of the orthophoto is that the orthophoto file offers more detail to plot the control points, back to the higher resolution of the photo. For example if it becomes necessary to add a control point where the primary school is, it would not be possible to use the DRG file because the location of the school would be hard to recognize. What transformation method will you choose when georeferencing the Sanborn maps, and why? The most suitable method for geo-referencing of the Sanborn image would be the Affine transformation, because in this case this method works with the images that requires rotation, transformation, different scaling and skewed operations. In this case the instructions require the rotation and scaling of the image, so the most suitable method will be the affine transformation. When georeferencing the Sanborn map image files, will the RMS Error reported by the software be of any more diagnostic value than your visual assessment of how well the Sanborn map aligns with the orthophoto? Explain. RMS error would not of diagnostic value in this case because the coordinate system of the Sanborn map is not known, since it is a painted map of the area. Therefore, the best way to define the error associated with the geo-referencing is by visual assessment. What specific steps will you take to create Shapefile datasets from your final feature classes? Remember, the Shapefile datasets must be in Geographic (lon-lat) coordinates. In other words, what specific ArcGIS tools or commands will you use? To extract the shapefiles from the feature classes I will need to select the feature classes and then export the data into shapefile sets that use the same coordinate system as the feature classes. After creating the shapefiles, the projection tool is used to convert geographic coordinates to longitude and latitude. Prior to georeferencing the Sanborn map, a number of steps must be completed in ArcCatalog which will be described below, together with the steps required in ArcMap, appropriately sequenced. The geo-referencing operation is estimated to take one to two hours. 1 – The first step is to define the spatial reference based map used for geo-referencing. In this case the spatial reference is UTM Zone 17 North, NAD83, which is assigned by using ArcCatalog or by using the definition projection tool in the arc tool box. The type of projection could either be selected from the projection folder or imported from the feature class that has the desired projection. The following pictures show a step by step explanation of the path to the design of the projecting in ArcCatalog. 2 – The second step is to import the title and the base map into the ArcMap (the base map should be imported first) and ‘accept’ the creation of a pyramid. 3 - Activate the geo-referencing toolbar from the Customizer tool and zoom to the base map layer from the table of contents. From the geo-reference tool select the title map for the (layer) and then click ‘Fit to Display’ to position the title over the base map. Uncheck the Auto Abject for this operation to avoid auto move of the image in the digitalizing process. 4 – The fourth step is rotate, shift or scale (exists on the geo-reference toolbar) the title map in the linkage so that it is possible to see the place of the tile on the base map. This is followed by the plotting of the control points for the common places between the two maps (start by selecting the control point from the title map). There should be at least 4 control points., distributed on the map and not clustered at one place. The use of the magnifier is helpful in this step because it allows zooming in on the features in the base map, making them very clear and assisting in accurately aligning the title map. 5 – Step four is to check the Route Mean Square in the link table. Choose an affine transformation type which is suitable for this project, and then check the Auto Adjust box to apply the transformation to enable viewing of the tile map in its correct place over the base map. 5 – The fifth step is to check the tile to ensure it is in the correct place, by applying transparency on it, so that it is possible to see the base map through the tile map and decide whether or not it needs to be changed. 6 - The last step is to update and then rectify the map so that the final product is available as a separate .tif map, with the same spatial reference (UTM Zone 17 North, NAD83). # Special digitizing settings/considerations for each of the feature classes For the Polygon feature class, the use of the magnifier window will increase the accuracy of the digitized building, whereas the auto complete polygon, overlap and snapping tools will align the buildings that share walls. It will be necessary to create a separate polygon for each story and for each building, with the ‘Merge’ command used in case it is necessary to combine two polygons. It will also be necessary to complete the ‘attributes’ for each polygon with metadata before creating another polygon to prevent mistakes In case of the Polyline feature class, the magnifier window is used to increase the accuracy of the digitized building. The street will be digitized from the lowest to the highest address number and the ;attributes; for each line need to be filled with metadata before creating others live, to prevent mistakes. Lines in the centre of the street will be created as required and the ‘constrain’ parallel and perpendicular will give the streets the required aspect. Read More