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Manual 1. Introduction
3. GIS data management 3.1 Stage 1: Key Criteria The framework for the project has already been established, with the selected adoption of parts of Phase one of the BWEA 'Best Practice Guidelines for Wind Energy Development' (BWEA 1994). To find and test the suitability of sites, the developer must consider two separate sets of criteria or factors. The guidelines present the initial site selection phase with components from two broad areas, Technical and Environmental. The Criteria from these are broad in scope. The Technical criteria concentrate on a number of elements linked to modelling approximate wind speeds, topographic site conditions and accessibility. The Environmental criteria include landscape considerations, planning restrictions and proximity/visibility factors associated with local populations. It is stated that, "…all…elements are inter-related and they should be considered as having an important influence on each other" (BWEA 1994) suggesting that the criteria should be applied together when the developer is carrying out initial and modified modelling assessment of potential sites. Both Technical and Environmental criteria will be used together in carrying out the IC and MC phases of the project. It is likely that the initial IC approach will use all listed criteria. It is then up to the student to decide (with some technical justification) as to which criteria to exclude in a modified MC phase. A key concept that applies to IC suggests that the developer should be considering the size of the site at this early phase in the investigation. This is established as a maximum capacity, whereas the MC approach is to select the largest site found during the exercise and then assess for 'commercial viability'. 3.1.1 Choosing Six Technical Criteria There are a number of key criteria which generally fall under the description of 'Technical'. These can broadly be grouped under the headings of topographic requirements (3), wind speed (1) and accessibility (2). These are described briefly below and then summarised as a number (6) of criteria which are classified as Technical and must be included in the modelling. Orientation
Although the Turbine Hubs can rotate to position the rotor blades into the wind, the optimum ground position of the turbines is dictated by the dominant wind direction. As this project will not be modelling the more complex on-site positioning, a generalised criteria is used. The remaining criteria constraints are chosen to represent the technical needs of the turbines, as they must operate at maximum potential output. Slope
This will place the site in a terrain range suitable for maximising the wind resource. Elevation
Wind Speed
Proximity to Road Network
Proximity to National Grid
3.1.2. Choosing Environmental Criteria There are other criteria which more specifically fall under the description 'Environmental'. The BWEA lists a greater number of constraints in the landscape category for the developer to consider, but these are all environmental issues. "The criteria we can consider for minimising environmental impact are of course very subjective, and will no doubt remain open to debate…, there is no complete definition of all the discernible factors and suitable limits for exclusion zones." (Anderson 1996) This quote is useful in actually allowing us some scope in our modelling. When modifying our approach from the IC to MC this is more likely to be where changes are made and criteria are dropped. These criteria can be grouped under headings of planning constraints (1), population safety (1) and environmental impacts (2) These are described below and then summarised as a further set (4) of criteria, classified as Environmental and which will be included in the modelling. Planning Restrictions
Population Safety & Impact Additionally, Noise intrusion is a significant factor in the siting of a wind farm. The process of assessment of the noise has been carried out by the Netherlands Environment Agency, who have set a maximum emission level of 40decibels, based on average wind speeds as, "…the emission - level of the turbine and the level of the background noise vary with the windspeed." The BWEA suggest that, "…the sound of a working wind farm is actually less than normal road traffic or an office" (BWEA 1994) and places the wind farm at 35-45 decibels (dB), between a quiet bedroom and a car travelling at 40mph at 100 metres away. It is suggested that for both safety and low noise intrusion, a wind farm should be no less than 500 metres from any residential areas. In this study the distance buffer is eliminated, as the distance from dwellings for safety has already been established at 500 metres, so noise would not therefore be a significant issue. So Criteria 8 then becomes,
Water Pollution
Interference "The rougher the surface, the more the wind…is impeded.,…Providing there are no obstacles ( i.e., buildings, trees or hills ), the wind speed at a given height is nearly the same over the entire area." (Hunt 1981) This invites a buffer zone from woodland, but the BWEA does not publish a recommended minimum distance. The New & Renewable Energy Bureau (ETSU) indicate that the buffer for woodland has been increased from 100 metres to 200 metres and above. To eliminate any possible interference from woodland, the criteria is therefore set initially at 250 metres. As the wind resource is a function of the terrain, elevation and slope, and an inspection of the Ordnance Survey map shows that there is very little woodland on the slopes of the South Downs, this criteria will not be modelled.
Summary of the Criteria to be applied
in the initial model:
Criteria to be included in Initial Modelling (IC). Technical and Environmental criteria are marked in bold as T and E respectively.
3.2 Stage 2: Data Collection
We now know the principal sets of criteria which we need to apply. How do we now go about collecting the data for our chosen area to begin to model these processes. This is a major stage of any GIS project and this is no exception. The manual will lead you through some of the key datasets involved and through the process for a specific area in the South-East near Brighton & Hove. It is your task to replicate and even develop the process as you do the same modelling for a part of the South-East you are interested in. Remember you must pick an area which has a reasonable elevation i.e. a minimum of 50 metres. You will not be allowed to use the same area developed within this manual. The other consideration at this point is to state that the data collection process will require different approaches and varied amounts of effort and time. This is also a point at which GIS skills will be used more closely to make the data match the GIS format. The Criteria will essentially fall into 3 main categories. These range from;
It is fairly clear that those criteria in category a) will require little time or processing, while criteria falling into category b) will need more processing and more time. The final criterion, d) is likely to take the most time and will require a laborious data entry process as well as deepening one's knowledge of the OS National Grid. Once entered into a format which can be read in a GIS it is a relatively quick process to create a wind speed surface. Category a) Criteria 7: SSSI's are included on the
CD/Disk for a selected block of the South-East. Category b) The second key element in the data collection process is access to the University of Edinburgh's Digimap site. As this case study is only available to registered institutions then you will be a student in a University that subscribes to the service. The service is a valuable agreement between subscribing Universities and the OS and gives you the ability to download all of the base digital topographic files you will need to carry out the GIS modelling. All individual students must be registered to use Digimap. Without this registration it is impossible to carry out the case study. PLEASE MAKE SURE YOU ARE REGISTERED. We will assist you in this process at the start of the Case Study if required. The next stage of data collection is to
use Digimap to download the specific digital tiles you want to build up
the criteria. In the case study you will be asked to download and convert
tiles for two separate OS digital products. These are; Downloading data from Digimap is relatively straight-forward. What is a little more complex is the conversion of that downloaded digital file into a format that ArcView can understand. Digimap provides files in a format known as National Transfer Format (NTF). This is a generic format which is not specific to any single GIS but requires a software-specific translation programme to convert into a format that the GIS can understand. This is because there are over 50 commercial GIS packages sold on the open market and it is not the OS's responsibility to make their data match every piece of software. Fortunately for the case study we have access to a programme called MapManager6.2 which directly translates NTF files into native ArcView Shape files. Instructions on how to both download data and translate it from NTF to Shape file format are included in Appendix MM2 - Mini-Manual 2 and will be explained further in the next section. Five of the criteria in this category are based on the digital layers downloaded and converted from Digimap As can be imagined, these criteria will require the use of the distance command on vector layers and then some raster querying to identify them. The first 3 criteria also demand the use of the Spatial Analyst extension. These are all processes which should be well within the scope of a second level GIS user. Category c) On your computer (or attached to WebSite/CD) is a zipped executable version of the ETSU NOABL Wind Speed Database. This is a free piece of software which is made available via the web and which students have full permission to use. The web-site from which it comes is listed in the Appendix. You will need this program to identify the data you need to create a wind speed surface for use in Criteria 4. This is a slow process which will involve a fair amount of manual processing as well as developing knowledge of the UK National Grid. We will only ask students to create a data set for 100 square kilometre area to minimise this amount of work though obviously developing data for a bigger area will aid in the siting process. It will involve using the Wind Speed Database to create a set of readings for 100 separate points with wind speeds at three separate heights 10m, 25m and 45m. It is also recommended that students extrapolate this data if they wish to model higher topography. The difficulty with the data is that it calculated from base surface level so working in the area we are in, with heights reaching 240 metres, will involve either assuming the same wind speeds at 50 and 250 meters or making an intelligent estimate of increased speeds on higher ground. This is explained in a little more detail in the manual in the Appendix MM1. 3.3 Stage 3: Data Collation Category A Downloaded and available shape file of SSSI's of the South-East: SESSSI. This will need to be converted to a GRID file to match the other criteria. In the case of this file this can be a little trickier than it looks as the key is to identify as a criteria only those areas which are not an SSSI and yet the vector file only shows those that are. The best thing to do is to the same as in category B file (see below for an example) by creating a distance file and then finding all values that are greater than 0 (to bring us right to the edge of the SSSI areas). This gives you the theme for Criteria 7. Category B There are two main digital file types to be downloaded from Digimap. The first of these will be Panorama tiles and the second, Meridian2 files. A set of tiles are suggested below for use in the workshops but remember you will have to select a different area when carrying out the case study yourself. Panorama Tiles TQ20 and TQ40. These can be downloaded as a zip file and then unzipped to produce NTF files. Panorama files are almost automatically converted within Map Manager (see Appendix MM2). The output will be in the form of 2 ASCII files - named TQ20.asc and TQ40.asc respectively. These can then be imported as data files (File> Import Data Source) and saved as two GRID raster layer within ArcView. By using the Grid Analyst extension (make sure this is ticked in the File > Extensions command) you can merge the two tiles using the Transform Grid > Merge command. To generate the criteria from the merged
DTM is a relatively simple process and should use existing knowledge of
Spatial Analyst. This will create Criteria 1, 2 and 3 by querying the
initial DTM as well as the derived Slope and Aspect layers. In the case
of both slope and aspect you will need to use the Map Query option and,
within that, the Boolean AND , box as this will allow you to type in the
lower and upper boundary values i.e. greater than 8 and less than 15 degrees
in the case of slope. Meridian2 Tiles TQ30, TQ31, TQ40 AND TQ41.
Having loaded up the layers from the converted Meridian tiles, it is now possible to run the Find Distance command to move towards creating some of the criteria you need. This manual will not spell out each one in turn but rather will describe a sample approach for one and let you work out the rest for yourselves. EXAMPLE: In the case of Criteria 8, where you need to identify all sites which are at least 500m from an urban area you will initially need to make the Urban.shp layer active, run the Analysis > Find Distance command which will then create a distance layer. You will then need to query the resultant raster output layer to identify those locations which fit that specific criterion, via the command Analysis > Map Query > Distance to urban.shp >= 500. This produces a Boolean output file with two values, 0 and 1, with the latter identifying those areas which are suitable for a wind farm according to that criteria. You should name this file in the Theme > Properties box as Criteria 8. The other criteria which fall into this category can be modelled in a similar way, giving you the remaining vector-derived layers, Criteria 5, Criteria 6, Criteria 9 and Criteria 10. Category C Run the WIND SPEED programme the data to match the tiles you have already identified. This process is described in Mini-Manual MM1 which explains the way in which the data is stored and gives you guidance on the way in which you need to set up and store the raw data. You will later import this into the GIS as a point layer. This is perhaps the trickiest data layer to create and will involve some basic data entry. Essentially you start by identifying an area you wish to get data for. We are using in this case a 10 X 10 kilometre area east of Brighton. We are restricting the area being modelled to this size as it will mean than the laborious semi-manual process of identifying and entering the wind speeds is minimised to 10 by 10 readings at three different heights. As the manual suggests, the key is to identify the correct grid co-ordinate for the bottom left hand corner point and then work up from there. In the case of the ETSU NOABL database it needs to have the data as a simple x and y co-ordinate with the appropriate two-letter code in front, i.e. TQ3010. You will later need this as a 6 digit co-ordinate to import into ArcView but this will be simple to do. Start by typing in the two letter code for the 100km square block, in this case, TQ. This will automatically put the appropriate numbers in for OS x and y co-ordinates. Then enter the remaining two-digit numbers for the easting and northing, in this case, 30 and 10 respectively. You will notice that the right hand set of boxes has converted TQ3010 into the form 530, 110 which is what the programme understands. Then click on the blue button to the right. We are then presented with the wind speeds for that point plus those of the 8 points around it for all three heights in three separate grids. In this way the data can be entered more quickly. The key is to understand and get the co-ordinates right. You will then need to do this for each of the other co-ordinates in the grid but this is simply a case of changing the references at the top and pressing the blue button again. This will appear very laborious but is perhaps intended as a lesson/Sisyphean task for all GIS users to show them the number of steps one has to go through to create clean data for the system when it is not already in GIS format. Once the data is collected from the Wind Speed programme, it needs to be created in the following format as a simple x,y,z text file. This is where a simple multiplication of the co-ordinate column by 1,000 can be done to make it match the GIS. X co-ord Y co-ord Wind Speed at 10m Wind
Speed at 25m This can then be imported to ArcView using the Add Table option within the Project window and then running the Edit > Add Event Theme command. A previous version of the text file derived from the Wind Speed database for the chosen area has been set up called Windv7.txt and we will use this in the workshops. This will create a point file which then needs to be immediately converted to a shape file (File > Convert to Shapefile). We now have a point file in native ArcView format which can be interpolated using Spatial Analyst at each of the chosen heights. For example to create a surface from this point file the Surface > Interpolate Points command may be used with IDW method and selecting the appropriate z value for any one of the columns of data in the file. This creates the wind surface at that height and will provide us with a key criteria for subsequent modelling. By querying this resultant wind surface layer with Map Query we can finally create our final Criteria 7. |
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