Lecture 11

[Music] hello everyone so in last lecture we have learned how we can download different kind of satellite data from different uh websites so we have learned how we can download the srtm data CED 1 data and L pulsor data and how we can use this data uh to interpret and uh get the more information from this data through the gis platform and we have also uh seen the some basic layout of the uh qgis that is a uh freely available software so today we will start with our second lab that is Geo referencing and we will see how we can do the uh ge referencing uh through the qgis as well as we will give a uh short uh uh description about the GE referencing through the argis so GE referencing is the process of assigning location to the geographical digital object within a geographical frame of reference so the images which we are getting through the uh satellite data or sometime the uh images which we are capturing through the our photographic camera so that uh it is very important to uh assign those photographs uh a geographic coordinate system that means the latitude and longitude so what is GE referencing GE referencing means yeah uh non- ge reference image is GE reference to an existing image that that is already assigned a geographic coordinate system and with the help of that uh ge reference image we can U assign a uh non- Geor reference image to the geographic coordinate system that means the latitude and longitude so it is a process of taking a digital image that could be your ear photo your scan geological map or a picture of topographic map and adding geographic information to the image so that GIS platform or mapping software can place the image in its appropriate real world location this process is completed by selecting pixels in the digital image and assigning them Geographic coordinates so how we do the GE referencing with the gis platform so GE referencing is accomplish by selecting a cells that is uh a grid with the DN values at the base image base image means that image have a known geographic coordinate system and finding its corresponding location in the Target image means Target image means the image which don't have the geographic coordinate system with the help of projection and then Computing the DN value for that point in the Target image by resampling Method so what are the projections and the resampling method and how we uh how the projection and the resampling method work uh by uh doing the coordinate transform that we will see in our coming slides so in general there are four steps to GE reference your data the first step is selection a base map or the image which have a known coordinate system in terms of latitude and longitude and then selection of gcps gcps are the ground control point the ground control points are the uh some common point in both the images which you can easily identify in uh both the images means the in your base image and your in your target image and with the help of these gcps or control points you can overlap this uh this both the image means the your target image on on the uh your base image and use the G reference tab the uh in your uh using your uh G software to create uh create the control point and connect your raser to known position in the map and then select the transformation method this transformation methods are select based on the how much you want to distort your image means uh uh if your image is initially having some uh assign some coordinate system and but it is associated with some kind of error and then you want to uh rectify the image in that case you have to you can apply a first order or second order of polinomial equations or transformation method I will tell you what are this first order or second order polinomial equations in the next slide and then you just uh assign the DN values to your targeted uh image and then review or matching the control points which you have chosen for both the images and the error these error are associated based on the your uh uh label of polinomial equations or the uh uh your uh uh the error Associated while choosing the control points and at last save the GE referencing result when you are satisfied with the alignment now at the control point so the the GE referencing process involves identifying a series of ground control points in in terms of x and y coordinate that is the latitude and uh uh longitude that link location on the raster data set with location in a especially Geor reference data control points are location that can be accurately identified on the raster data set and in real world coordinates many different type of features can be used as identifiable locations such as Road or streams intersections the mouth of a stream rock out crop the end of JY of land the corner of established Field Street Corner or intersection of two head rows so these are some of the uh points which you can use while choosing your ground control points so in this image you can see this is your ground control Point see this this red circle this red circle is your uh it is a corner of a two plot and you can easily identify this corner in in your target image and and your base image so you can choose these Corners as your ground control points similarly here the second Ground Control point this is a a house or and if you are able to identify this house in both the images so you can choose this house as your ground control Point similarly in the second image you can see this is the corner of a house or a building and this point you can choose as a uh your ground control point so simil simil Sly you can also choose a uh intersection of a road where two roads are uh Crossing each others suppose this is one road and your another road is passing from here so this intersection point you can choose as your ground control point so these are some of the factor how you can choose your ground control point and the basic concept is that uh you you you would easily able to identify this Ground Control control point in your base image and your target image so the transforming uh transforming the roster image so when you are uh ge referencing your uh Target image with the help of your uh base image that is already GE reference so there is a coordinate trans transformation from the uh base image to the Target image so this coordinate transformation is done through the U some Pol omal equations this polinomial equations are assigned based on the uh uh based on the order of distortion you you are doing while transforming the coordinate system from uh Target image to uh from base image to Target image so the polinomial transformation uses a polinomial build on control point and at least Square fitting algorithm the lower order polinomial tend to give a random type of error while while the higher order polinomial tend to give an extrapolation error that means that if you want to Simply rotate or displace your image slightly so in that case you have to just you can apply the lower order polinomial equations for example the zero order polinomial equations or the first order polinomial equations and if your uh your image is uh distorting while uh doing the coordinate transformation in that case you have to apply the higher order polinomial equations to do the coordinate transformation uh and GE referencing the number of non-correlated control Point required for this method must be one for zero order polinomial equations and three for a first order polinomial equation or aine uh coordinate transformation uh you have to choose the six control point for a second order polinomial equations and at least 10 Ground Control point for a third order polinomial equations so keep in mind that the higher order polinomial equations you want to apply you need to add uh more uh Ground Control point to do the GE referencing a zero order polinomial is used to shift your data this is commonly used when your data is already Z reference but a small shift will better line up your data only one control point is required to perform a zero order polinomial shift here in this image you can see this is suppose this is your original data and you just want to slightly modify or shift your data in that case you can apply the first order or the aine polinomial equations and if you if you want to uh if your data is associated with some higher order of distortion in that case you have to apply the higher order of polinomial equations in this uh in this case you have to apply the second order polinomial equations and for the uh more Distortion data you have to apply the higher order uh polinomial equation that might be the third order polinomial equations so here uh use a first order or aine trans transformation to shift scale and rotate a raster data set this generally results in straight line on the raster data set mapped as a straight line in the B raster data set the higher the transformation order the more complex the Distortion that can be corrected the second method is after coordinate transformation you have to assign the DN values to your uh your uh uh pixels in the Target image so this is done through the resampling or interpolation method so how uh this is this can be done here we can see so three type of resampling method are used to assign the DN values to a pixel so first is your nearest neighbor so what happens when suppose this is your this is your target this is your target image and this is your this is your base image so it has associated with some uh DN uh pixels and it is associated with some DN values and you you have just Co registered with you with the coordinate system now you want to assign the DN value so what will happen during the uh uh with the help of polinomial equations you have registered uh this image and or overlap this both these image images so now if suppose you want to assign uh this Grid or this pixel your DN Valu so the in in the nearest neighbor method what will happen the uh uh this this pixel will uh get the DN values the targeted image pixel will get the DN values from the base image from its whichever pixel is nearest to this this pixel when you overlap both these images through the polinomial equations so this this pixel or this grid it will get the DN values from its uh nearest Grid or pixel so that is your nearest neighbor the point in the new grid simply acquire the DN values of that point in the older Grid or the or the base image which lie closest to it and the second method of uh second resampling method is your bilinear interpolation in the bilinear interpolation the pixel is simply getting the DN values from its uh from the four pixels which is closest to it that means suppose this is your this is your pixel or grid of the target image so this pixel or grid will get the DN values from the surrounding four pixel or four grid so suppose the these are the four grids from your base image and this this pixel or grid will get the DN values from this four pixels so this method is called the bilinear interpolation method and in the cubic convolution when your target image pixel is getting DN values from the surrounding 16 pixels or grid so in that case you this in this this method is called your cubic convolutions so after doing the coordinate transformation and assigning DN values you have to Simply look for the root mean square error so that is the error which is associated with the uh ge referencing method or or the your uh polinomial equations or the uh choosing the ground control point so that you have to look for it when the joural formula is drived and applied to the control point a measure of residual error is return the error is the difference between where the point ended up as opposed to the specified locations the total error is computed by taking the root mean square that is RMS sum of all the residuals to compute the RMS error this value describe how consistent the transformation is between the different control points when the error is particularly large you can remove the remove and add add control point to adjust the error so now we will so now we will see how we can do the uh the GE referencing with the help of argis here I will explain uh it uh how we can uh ge reference a topo sheet on the arc GIS but we will give a demo uh to GE reference a topo sheet on the qgis because as in our last lectures we have uh uh install the qgis and we have also uh gave a demo on the qgis so we will uh uh show you how we can do the GE referencing uh on the qgis as well as the argis so argis as uh in our last lecture I uh described that this is also a GIS platform where you can do all kind of mappings so uh when you will open the argi you can see such type of window will open here uh similarly the uh here you can see the file edit uh insert selection tool window it it's a Sim similar kind of window which you have looked in your uh qgis and here to open your data you will simply here you can see a plus sign with uh clicking this plus sign you you can add your data so here we we will show you how we can uh ge reference a topographic map so first we have to open the topog uh uh uh Topo sheet with the help of this plus sign and here in this layer option you will you can see all the layers or the files which you have opened so here we have open a topo sheet the Topo sheet name is l 3479 and it is in a JPEG format so here you can see in this layer this image is open and you have to uh do the G referencing uh you have to GE reference this uh Topo sheet so first thing you you have to keep in mind then when you you are opening a image first you have to check whether this image is Geo reference or not that you can easily see here this in this section on the uh argis platform here you will see the there is some values are written that is in negative or the unit section it is written the unknown unit that means this image is uh uh is not GE reference or this image is not assigned any coordinate any coordinate in form of latitude and longitude so in the second step to uh G reference your Topo sheet first you have to zoom your Topo sheet at the corner side when you will Zoom your Topo sheet you can see that your longitude and your latitude is written on the Topo sheet so for example this Topo sheet this the upper left corner of this Topo sheet is a 74° and 30 minute and the latitude is 90° 0 minute so to do the GE referencing you have to go to the uh first you have to uh go to the view section and in this view section uh uh in the uh panel option you have to first enable the GE referencing here you can see this is the GE referencing panel and if this ge referencing panel is not uh enable then you have to go to the view and then from there you can enable the GE referencing option once this ge referencing option is enabled you can you will see the GE referencing panel over here and here you can see that the your open map is also you can see here this is your map which you can Geor reference with the help of your uh GIS AR GIS to do the GE referencing first you have to select for the control point to add the control point you have to just go to this option this uh this option will direct you to the add the control point when you will click the uh this option and this uh a popup will come up the the enter coordinates and when you will this coordinate is for this point so when you click this point and you enter this point in X and Y that means latitude and longitude when you will enter latitude and longitude in X and Y and you will click okay so this will Mark in your M on your map and similarly you have to mark all the four corners while uh doing the uh uh scroll your mouse or zoom up that particular section and you have to mark all these all these four corner and you have to add the four control point with the help help of this four control point you will you will be able to uh get the gcps and to uh uh to analyze or review your gcps or the uh Associated error you can see you can click over here and this will open a table and on the table you can see or review all your point here you can see this is your this is your point which you you have marked based on your Topo sheet so this this uh here you can see this is 4 point and is XY and this is your residual so the residual is your Associated error with that uh individual control point for example the for the first point your Associated error is 14.9 and then 14.9 14.9 this is your Associated error for all the four point and at the bottom corner you can see this is your total Associated error the RMS and here you can see for this uh for this uh uh Topo sheet here we have chosen the first order polinomial equation so as I already uh describe that when the your uh dist uh uh total Distortion is minimum in that case you can apply the low order polinomial equations and if the error is higher then in that case you have to apply the higher order polinomial equation so after selecting all the points or uh all the gcps you you can uh directly go to the GE referencing option and you can click over there so you will get some of more option here you can see the update GE referencing Rectify Flip or rotate transformation auto adjust update display delete control point and reset trans formation so these are the option where you can Al you can change the transformation if you want to uh apply the higher order polinomial equations you can delete your control Point suppose you are getting the higher amount of uh RMS for a particular point you can delete that point with this option and you can again add a new control point with a minimum eror error and after doing all these uh practices and after selecting all the Ground Control point you can just click the update GE referencing and this will GE reference your error you and this will uh ge reference your image so while your image is GE reference now you have to save your GE reference image to save you just go to the save as option and here you can see few options are given the resampling method so the resampling as I already describe you that the resampling is assigning the DN values so what kind of resampling method is suitable for your particular data that you can Shuffle or you can check through this uh this down arrow here you you can you will be see the nearest neighbor the bilinear and your cubic convolution so you can uh you can uh uh Shuffle these uh options and you can see which one is uh uh giving you a more accurate GE reference data that you can choose for your data and then you here are few options are given the file name and image format in which format you want to get your GE reference data that you can choose and finally you can save your data and you can open your when you will open your data in your GIS platform so this this image or your uh file is now GE reference how you you would know that your image is GE reference here you can see at the uh down corner at the right down side of your screen you will see this is your latitude and longitude is written over here so now this image is GE reference so now we will move to the qgi software and over there we will give you a demo that how uh we will uh Geor reference a topo sheet first with the uh with the the help of the qgis and then we will also give you uh a demo on the uh a uh image which is uh uh not GE reference with the help of the base map so there is two method one is uh your uh in first method I have just described that how you can GE reference a topo sheet where uh the latitude and longitude is already given and another method is uh when your image uh uh is uh uh there is no latitude and longitude mention over uh on your image in that case you have to uh use a base map that is already Geor reference and how you will Geor reference your uh Target image with the help of that base image so I will give you the both the uh uh both the examples with the help of the qgis so in next lecture we will see how we can uh uh Geor reference the images on the qgis platform thank [Music] [Music] 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